Add dr-thumbs: a sharded, syncable thumbnail store

A thumbnail is the one derived artefact worth sending over the wire: it costs
a range fetch plus a decode to produce and is identical for every client
looking at the same file. A second device that downloads a shard gets a full
grid without fetching a byte of RAW.

Sharded at 25 MB, filled sequentially. The cap is about sync granularity, not
SQLite's limits — one growing database means every client re-downloads it
whenever a single thumbnail is added, whereas with sequential fill only the
newest shard is ever dirty and sealed shards are safe to cache forever.

Stored JPEG-encoded rather than as raw RGBA: a 256px RGBA buffer is ~256 KB
against ~20 KB encoded, and that 13x is transfer cost on every client.

Keyed on Nextcloud's oc:fileid, stable across server-side rename and move.

Assisted-by: LLM

Cargo.toml

@@ -3,6 +3,7 @@ resolver = "2"
 members = [
     "core/dr-types",
     "core/dr-catalog",
+    "core/dr-thumbs",
     "core/dr-decode",
     "core/dr-gpu",
     "core/dr-lens",
@@ -26,6 +27,7 @@ repository = "https://github.com/dtourolle/DarkRoom"
 # Internal
 dr-types = { path = "core/dr-types" }
 dr-catalog = { path = "core/dr-catalog" }
+dr-thumbs = { path = "core/dr-thumbs" }
 dr-decode = { path = "core/dr-decode" }
 dr-gpu = { path = "core/dr-gpu" }
 dr-lens = { path = "core/dr-lens" }
@@ -40,6 +42,13 @@ wgpu = "23"
 slint = { version = "1.9", default-features = false }
 slint-build = "1.9"
 
+# UI token codegen (S2): style.yaml -> theme.slint. serde_yaml was deprecated
+# by its maintainer in 2024 and serde_yml, the first fork, has since been
+# deprecated too; serde_norway is the fork still receiving releases. Its
+# mappings preserve insertion order, which is what lets the generated Slint
+# keep the token ordering the YAML author chose.
+serde_norway = "0.9"
+
 # Foundations
 anyhow = "1"
 thiserror = "2"
@@ -87,6 +96,11 @@ rusqlite = { version = "0.40", features = ["bundled", "backup"] }
 
 rawler = "0.7"
 zune-jpeg = "0.4.21"
+# Thumbnails are stored encoded, not as raw RGBA: a 256px RGBA buffer is
+# ~256 KB against ~20 KB as JPEG, and the store syncs to Nextcloud where that
+# 13× is transfer cost on every client. Pure Rust, no C dependency — the same
+# criterion behind the TLS and SQLite choices above.
+jpeg-encoder = "0.7"
 bytemuck = { version = "1", features = ["derive"] }
 
 # Lens correction profiles. A pure-Rust port of Lensfun rather than a binding