feat: in-game theme store — server catalog + verified downloads + install UI

Phase 1+2 of the theme-store roadmap: a free catalog served by
solitaire_server and an in-app browse/install flow, making custom
themes installable on Android for the first time (the manual
drop-a-zip flow can't reach the app-private themes dir there).

- solitaire_sync: ThemeCatalogEntry/ThemeCatalogResponse wire types
  (additive module; SyncPayload and SyncProvider untouched)
- solitaire_server: THEME_STORE_DIR scan at startup (meta-only
  theme.ron parse, sha256, 20 MiB cap, best-effort per archive);
  public GET /api/themes, /api/themes/{id}/download, /{id}/preview;
  compose volume + README_SERVER docs
- solitaire_data: ThemeStoreClient — catalog fetch + download with
  mandatory size/sha256 verification before bytes are released
- solitaire_engine: ThemeStorePlugin — 'Browse theme store' button in
  Settings → Cosmetic, modal catalog (leaderboard-style rebuild),
  download on AsyncComputeTaskPool, atomic .tmp+rename write into
  user_theme_dir, then the existing hardened import_theme pipeline and
  an in-place registry refresh

New deps: sha2 (workspace, server+data); ron/zip reused in server;
serde_json added to solitaire_sync dev-deps for DTO round-trip tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_data/Cargo.toml

@@ -24,6 +24,9 @@ uuid           = { workspace = true }
 dirs           = { workspace = true }
 reqwest        = { workspace = true }
 tokio          = { workspace = true }
+# Theme-store downloads are verified against the catalog's SHA-256
+# before the archive is handed to the engine's theme importer.
+sha2           = { workspace = true }
 
 # `keyring-core` is the typed Entry/Error API used by
 # `auth_tokens`. The crate's own dependency tree pulls in
@@ -47,6 +50,10 @@ sqlx             = { workspace = true }
 jsonwebtoken     = { workspace = true }
 uuid             = { workspace = true }
 chrono           = { workspace = true }
+# theme_store_round_trip builds a theme zip in a tempdir for the
+# in-process store server.
+zip              = { workspace = true }
+tempfile         = { workspace = true }
 
 [lints]
 workspace = true