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>

README_SERVER.md

@@ -44,6 +44,26 @@ docker compose up -d
 ```
 
 
+## Theme store
+
+The server can offer card-art themes for in-game download. Drop theme
+`.zip` archives (the same format the game's Settings → Import accepts:
+a `theme.ron` manifest plus 53 SVGs) into the directory named by
+`THEME_STORE_DIR` (default: `theme_store/` next to the binary), then
+restart the server — the catalog is scanned once at startup. An
+optional `<theme-id>.png` in the same directory becomes the theme's
+store preview.
+
+Endpoints (public, no auth):
+
+- `GET /api/themes` — catalog JSON (id, name, author, size, sha256)
+- `GET /api/themes/<id>/download` — the archive
+- `GET /api/themes/<id>/preview` — the preview PNG, if present
+
+Archives that are oversized (> 20 MiB), unreadable, or have a
+malformed `theme.ron` are skipped with a warning in the server log;
+they never fail startup.
+
 ## Admin — Password Reset
 
 If a player loses access to their account, the server binary includes a