feat(updater): in-app update on desktop, releases link on Android

Anyone who installed an AppImage or ran the Windows installer was frozen
on that version forever. Nothing in the app ever mentioned a new release
existed, and the release notes were the only announcement.

Desktop now checks a signed manifest, shows the version and its notes in
Settings, and installs and relaunches on request. The signature check is
the whole point: it is what stops a substituted download from being
installed by the app itself. Windows binaries stay unsigned for
SmartScreen purposes -- that is a code-signing certificate, a separate
problem -- but the update payload is verified against our own key.

Android is deliberately not wired to the updater. An app may not replace
its own APK; that is the package installer's job, and the plugin has no
Android implementation. It gets a link to the releases page instead of a
button that would throw.

The plugins are gated with a target-triple cfg rather than
cfg(desktop). Cargo only evaluates target cfgs in a [target.'cfg(..)']
table, so cfg(desktop) matches nothing, silently drops the dependency,
and fails much later with "Permission updater:default not found" -- which
is exactly what the first attempt here did.

Where the manifest lives took some finding. This Gitea serves
/releases/download/<tag>/<asset> but 404s on
/releases/latest/download/<asset> (verified against a real asset), so
there is no stable latest-release URL. The gitea-pages branch is
force-pushed wholesale by publish-docs.yml, so it cannot host the file
either. latest.json therefore gets its own orphan branch, read over the
raw-file URL, and is published from a scratch repo in RUNNER_TEMP rather
than by switching branches in the checkout -- doing that would have left
the following steps standing on a one-commit history, and the next step
but one runs release:notes against the real commit range.

Also fixed, all of it release-integrity:

  - "appimage" is in bundle.targets. The release notes have advertised an
    AppImage for months; tauri.conf.json never built one, the artifact
    step globbed for *.AppImage, found nothing, and said nothing. The
    step now fails instead.
  - The .AppImage.tar.gz/.sig pair and the NSIS .sig are collected. A
    manifest referencing a signature that was never uploaded fails only
    on the user's machine, so the manifest step also refuses to write an
    entry with an empty signature.
  - Release notes are generated by release:notes from the traceability
    graph, which is what CLAUDE.md has asked for all along, instead of a
    fixed heredoc that said "see CHANGELOG.md for detailed changes" and
    linked "GitHub Issues" on a Gitea-hosted project.
  - The notes tell users how to verify a download with SHA256SUMS.

Requirements UR-077 / DR-217, tests UT-208 (12 cases over the version
comparison and the platform decision, including that a pre-release does
not offer itself as an upgrade to the matching release).

Verified: 1070 frontend tests, cargo check for both the host and
aarch64-linux-android (confirming the plugins are absent there), clippy
-D warnings, svelte-check 0 errors.

src-tauri/tauri.conf.json

@@ -31,11 +31,23 @@
       }
     }
   },
+  "plugins": {
+    "updater": {
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IDhBMEY0NDJDRDAxRUU3NkMKUldSczV4N1FMRVFQaXNXSlV6U3RXdk5qT2NCY0s2eTZ2Q3RYS25MNnNKY09HcU5LSTJjUUx3d3MK",
+      "endpoints": [
+        "https://gitea.tourolle.paris/dtourolle/jellytau/raw/branch/updater/latest.json"
+      ],
+      "windows": {
+        "installMode": "passive"
+      }
+    }
+  },
   "bundle": {
     "active": true,
     "targets": [
       "deb",
       "rpm",
+      "appimage",
       "nsis"
     ],
     "icon": [