diff --git a/.gitignore b/.gitignore
index ba2478f..69321fb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,10 @@ Cargo.lock.bak
 # tools/film-profiles/convert.py --fetch. Not source: the converted
 # profiles in core/dr-film/profiles are.
 tools/film-profiles/upstream/
+
+# flatpak-builder's cache and its output tree. `packaging/flatpak/` holds the
+# manifest, which is source; everything a build derives from it is not — and
+# `.flatpak-builder/` in particular caches an unpacked copy of the whole
+# checkout, so it is larger than the repository it sits in.
+/.flatpak-builder/
+/build/
diff --git a/packaging/flatpak/paris.tourolle.darkroom.yml b/packaging/flatpak/paris.tourolle.darkroom.yml
new file mode 100644
index 0000000..c6b9c27
--- /dev/null
+++ b/packaging/flatpak/paris.tourolle.darkroom.yml
@@ -0,0 +1,186 @@
+# Flatpak manifest — FR-PLAT-LIN-3 (sandboxed distribution), NFR-COMPAT-2.
+#
+# YAML rather than JSON because this file has more to explain than to declare,
+# and JSON cannot hold a comment. flatpak-builder reads both.
+#
+# Build it, from the repository root:
+#
+#     flatpak-builder --user --install --force-clean \
+#         build/flatpak packaging/flatpak/paris.tourolle.darkroom.yml
+#
+# Read docs/distribution.md before changing any permission below. Every line in
+# `finish-args` is a hole in the sandbox, and the one that is conspicuously
+# absent — `--filesystem=` — is absent on purpose and is explained there.
+id: paris.tourolle.darkroom
+
+# 25.08 is the current freedesktop runtime, and the choice is made by what the
+# binary needs rather than by what is newest. `ldd` on a release build names
+# fontconfig, freetype, expat, libpng, zlib, brotli and bzip2 — all in the
+# Platform — and nothing else: Vulkan, libxkbcommon and the two display-server
+# protocols are reached without a link-time dependency (wgpu dlopens
+# libvulkan.so.1, and x11rb and wayland-client speak the wire protocols in Rust
+# rather than binding libxcb or libwayland). So the runtime has to supply a
+# Vulkan loader and an ICD at *runtime*, which is the GL extension's job, and
+# not much else.
+runtime: org.freedesktop.Platform
+runtime-version: '25.08'
+sdk: org.freedesktop.Sdk
+
+# The Rust toolchain is an SDK extension rather than something this manifest
+# installs, so the build is offline-capable in the part that matters and the
+# compiler is the one freedesktop tested against its own glibc.
+#
+# Note what this quietly overrides: `rust-toolchain.toml` pins 1.92.0, and that
+# pin is honoured by *rustup*, which is not what the extension provides. The
+# extension's cargo therefore ignores the file and builds with its own stable
+# (1.98.0 on 25.08). That is fine here and deliberately different from
+# CONTRIBUTING.md's "do not override the toolchain": the pin exists so `cargo
+# fmt --check` and `clippy -D warnings` agree between a laptop and CI, and
+# neither runs in this build. A *release binary* only needs a compiler at or
+# above the workspace's `rust-version`.
+sdk-extensions:
+  - org.freedesktop.Sdk.Extension.rust-stable
+
+command: darkroom-desktop
+
+finish-args:
+  # FR-PLAT-LIN-2 asks for both display servers. `fallback-x11` rather than
+  # `x11`: it grants the X socket only when Wayland is unavailable, so a
+  # Wayland session does not leave an X11 hole open beside the socket actually
+  # in use. `--share=ipc` goes with it — without it X11 cannot use shared
+  # memory and every frame is pushed through the socket instead.
+  - --socket=wayland
+  - --socket=fallback-x11
+  - --share=ipc
+
+  # The GPU. The develop pipeline is compute shaders through wgpu and there is
+  # no CPU renderer behind it, so this is not an optimisation: without
+  # /dev/dri the application starts and cannot develop anything.
+  #
+  # `dri` rather than `all`: it covers the render nodes and the NVIDIA device
+  # nodes, which is the whole of what a Vulkan ICD opens. `all` would add every
+  # other device on the machine for no gain.
+  - --device=dri
+
+  # Nextcloud (FR-NC-*). Nothing else here reaches the network — face grouping,
+  # segmentation and lens correction are all local and stay local (NFR-SEC-5).
+  - --share=network
+
+  # Credential storage (FR-NC-2). The keyring crate speaks the Secret Service
+  # D-Bus interface directly, which GNOME Keyring and KWallet's `ksecretd` both
+  # implement, so what it needs is a talk hole to that well-known name.
+  #
+  # Worth being precise, because FR-PLAT-LIN-3 says "Secret Service portal" and
+  # these are two different things: xdg-desktop-portal's `org.freedesktop.
+  # portal.Secret` hands an application a master key for a store it keeps
+  # itself, whereas this talks to the session's secret daemon. Only the latter
+  # puts the app password where `secret-tool` and Seahorse can see it, which is
+  # what makes a credential individually revocable by the user rather than
+  # opaque inside our own data directory. If no daemon answers, FR-NC-2's
+  # degraded mode is what the user gets — the same behaviour as outside a
+  # sandbox, which is the point.
+  - --talk-name=org.freedesktop.secrets
+
+  # No `--filesystem=` line of any kind, and this is the substance of
+  # FR-PLAT-LIN-3 rather than an omission.
+  #
+  # What that leaves working: /run/user/$UID/doc is mounted in every sandbox, so
+  # a photograph opened from a file manager — the .desktop entry declares the
+  # RAW MIME types and `Exec=darkroom-desktop %F` — arrives as a document-portal
+  # path in argv and opens. That path is genuinely portal-mediated and needs no
+  # code change.
+  #
+  # What that leaves broken: choosing a *library root*. The folder connector
+  # takes a typed absolute path (`SignIn::EndpointOnly`, placeholder
+  # `/home/you/Pictures`) and checks it with `std::fs`, and nothing in the tree
+  # calls the FileChooser portal — there is no ashpd, no rfd, no toolkit dialog.
+  # A path typed into that field does not exist in this sandbox, so the launch
+  # screen refuses it with "that folder does not exist", which is at least an
+  # honest error.
+  #
+  # `--filesystem=host` would make that work today and is exactly what the
+  # requirement forbids, so it is not here. docs/distribution.md §4 records what
+  # closes the gap and how to run a Flatpak build in the meantime.
+
+modules:
+  - name: darkroom
+    buildsystem: simple
+
+    build-options:
+      append-path: /usr/lib/sdk/rust-stable/bin
+      env:
+        # Inside the build sandbox rather than in $HOME, so a rebuild starts
+        # from the state flatpak-builder is managing and not from whatever the
+        # host's cargo cache happens to hold.
+        CARGO_HOME: /run/build/darkroom/cargo
+      # Cargo fetches 826 crates, and Flathub's builders forbid this — a
+      # submission there needs `cargo-sources.json` generated by
+      # flatpak-builder-tools' `flatpak-cargo-generator.py` from Cargo.lock,
+      # listing every crate as its own source, plus a vendored-registry
+      # `.cargo/config.toml`. That file is ~30k lines, has to be regenerated on
+      # every dependency change, and buys nothing for a build from a local
+      # checkout, which is what this manifest is for and what packaging/PKGBUILD
+      # is for as well. Add it when there is a Flathub submission, not before.
+      build-args:
+        - --share=network
+
+    build-commands:
+      # `--locked` for the reason CI uses it: a lockfile that resolves
+      # differently in the packaging build than in the tree is a release whose
+      # dependency versions nobody chose.
+      - cargo build --release --locked -p darkroom-desktop
+
+      - install -Dm755 target/release/darkroom-desktop /app/bin/darkroom-desktop
+
+      - install -Dm644 packaging/paris.tourolle.darkroom.desktop
+        /app/share/applications/paris.tourolle.darkroom.desktop
+
+      - install -Dm644 packaging/paris.tourolle.darkroom.metainfo.xml
+        /app/share/metainfo/paris.tourolle.darkroom.metainfo.xml
+
+      # The icon's name is the contract, not its path — the desktop entry says
+      # `Icon=paris.tourolle.darkroom` and the shell resolves that through the
+      # hicolor theme. 256x256 because that is the source's actual size;
+      # installing it under a size it is not makes scaled icons look wrong.
+      - install -Dm644 ui/dr-ui/ui/app-icon.png
+        /app/share/icons/hicolor/256x256/apps/paris.tourolle.darkroom.png
+
+      # The face models, where `system_face_models_dirs()` looks: it reads
+      # $XDG_DATA_DIRS, which includes /app/share inside a Flatpak, so this is
+      # the same lookup that finds /usr/share/darkroom/models from the Arch
+      # package. Last in the search order, so a pair the user dropped in their
+      # own data directory still outranks these.
+      #
+      # These live in Git LFS. A checkout made without `git lfs pull` has
+      # ~130-byte pointers here, and `type: dir` below would copy the pointers
+      # in without complaint — producing a Flatpak whose face indexing fails
+      # inside the graph loader on the user's machine. Refuse instead, with the
+      # command that fixes it.
+      - |
+        for m in scrfd_500m_640.onnx arcface_mbf_b1.onnx; do
+          if [ "$(stat -c%s "models/face/$m")" -lt 100000 ]; then
+            echo "error: $m is an LFS pointer, not a model — run: git lfs pull" >&2
+            exit 1
+          fi
+          install -Dm644 "models/face/$m" "/app/share/darkroom/models/$m"
+        done
+
+      - install -Dm644 README.md /app/share/doc/darkroom/README.md
+
+    sources:
+      # The local checkout, for the same reason packaging/PKGBUILD builds from
+      # one: this makes a Flatpak of what you are actually working on. Swap it
+      # for an `archive` or `git` source with a tag when there is a release to
+      # point at.
+      #
+      # `skip` is not tidiness. `target/` is tens of gigabytes and `.git` with
+      # LFS objects is not small; flatpak-builder copies a `dir` source
+      # wholesale, so without these two lines the copy is the slowest part of
+      # the build by a wide margin.
+      - type: dir
+        path: ../..
+        skip:
+          - target
+          - target-android
+          - .git
+          - build