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_server/tests/server_tests.rs

@@ -1614,6 +1614,7 @@ async fn auth_rate_limit_returns_429_on_11th_request() {
     let state = solitaire_server::AppState {
         pool: test_pool().await,
         jwt_secret: TEST_SECRET.to_string(),
+        theme_store: std::sync::Arc::new(solitaire_server::theme_store::ThemeStore::empty()),
     };
     let app = solitaire_server::build_router(state);
 
@@ -1675,6 +1676,7 @@ async fn sync_push_rate_limit_returns_429_on_11th_request() {
     let state = solitaire_server::AppState {
         pool: test_pool().await,
         jwt_secret: TEST_SECRET.to_string(),
+        theme_store: std::sync::Arc::new(solitaire_server::theme_store::ThemeStore::empty()),
     };
     let app = solitaire_server::build_router(state);
 
@@ -1879,3 +1881,125 @@ async fn replay_upload_malformed_body_returns_400() {
     let resp = post_authed(app, "/api/replays", &token, bad).await;
     assert_eq!(resp.status(), StatusCode::BAD_REQUEST);
 }
+
+// ---------------------------------------------------------------------------
+// Theme store
+// ---------------------------------------------------------------------------
+
+/// Writes a minimal theme zip (manifest only — the catalog scan reads
+/// just the `meta` block) into `dir` and returns its path.
+fn write_store_zip(
+    dir: &std::path::Path,
+    file_name: &str,
+    id: &str,
+    name: &str,
+) -> std::path::PathBuf {
+    use std::io::Write as _;
+    let manifest = format!(
+        r#"(
+    meta: (
+        id: "{id}",
+        name: "{name}",
+        author: "Test Author",
+        version: "1.0.0",
+        card_aspect: (2, 3),
+    ),
+    faces: {{}},
+    back: "back.svg",
+)"#
+    );
+    let path = dir.join(file_name);
+    let file = std::fs::File::create(&path).expect("create zip");
+    let mut writer = zip::ZipWriter::new(file);
+    let options = zip::write::SimpleFileOptions::default();
+    writer.start_file("theme.ron", options).expect("start_file");
+    writer
+        .write_all(manifest.as_bytes())
+        .expect("write manifest");
+    writer.finish().expect("finish zip");
+    path
+}
+
+/// `GET /api/themes` returns the scanned catalog as JSON, no auth needed.
+#[tokio::test]
+async fn theme_catalog_lists_scanned_themes() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let zip_path = write_store_zip(dir.path(), "neon.zip", "neon", "Neon");
+    let store = solitaire_server::theme_store::ThemeStore::scan(dir.path());
+    let app = solitaire_server::build_test_router_with_theme_store(test_pool().await, store);
+
+    let req = Request::builder()
+        .uri("/api/themes")
+        .body(Body::empty())
+        .expect("build request");
+    let resp = app.oneshot(req).await.expect("oneshot");
+    assert_eq!(resp.status(), StatusCode::OK);
+
+    let body = axum::body::to_bytes(resp.into_body(), usize::MAX)
+        .await
+        .expect("read body");
+    let catalog: solitaire_sync::ThemeCatalogResponse =
+        serde_json::from_slice(&body).expect("parse catalog");
+    assert_eq!(catalog.themes.len(), 1);
+    let entry = &catalog.themes[0];
+    assert_eq!(entry.id, "neon");
+    assert_eq!(entry.name, "Neon");
+    assert_eq!(entry.download_url, "/api/themes/neon/download");
+    assert_eq!(entry.preview_url, None);
+    let zip_bytes = std::fs::read(&zip_path).expect("read zip");
+    assert_eq!(entry.size_bytes, zip_bytes.len() as u64);
+}
+
+/// The download endpoint streams back exactly the bytes on disk, so a
+/// client hashing the response gets the catalog's sha256.
+#[tokio::test]
+async fn theme_download_returns_archive_bytes() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let zip_path = write_store_zip(dir.path(), "neon.zip", "neon", "Neon");
+    let store = solitaire_server::theme_store::ThemeStore::scan(dir.path());
+    let app = solitaire_server::build_test_router_with_theme_store(test_pool().await, store);
+
+    let req = Request::builder()
+        .uri("/api/themes/neon/download")
+        .body(Body::empty())
+        .expect("build request");
+    let resp = app.oneshot(req).await.expect("oneshot");
+    assert_eq!(resp.status(), StatusCode::OK);
+    assert_eq!(
+        resp.headers()
+            .get("content-type")
+            .and_then(|v| v.to_str().ok()),
+        Some("application/zip")
+    );
+    let body = axum::body::to_bytes(resp.into_body(), usize::MAX)
+        .await
+        .expect("read body");
+    assert_eq!(&body[..], &std::fs::read(&zip_path).expect("read zip")[..]);
+}
+
+/// Unknown ids 404 on both file endpoints; a theme without preview art
+/// 404s on `/preview` while still serving its download.
+#[tokio::test]
+async fn theme_unknown_id_and_missing_preview_return_404() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    write_store_zip(dir.path(), "neon.zip", "neon", "Neon");
+    let store = solitaire_server::theme_store::ThemeStore::scan(dir.path());
+    let app = solitaire_server::build_test_router_with_theme_store(test_pool().await, store);
+
+    for uri in [
+        "/api/themes/nope/download",
+        "/api/themes/nope/preview",
+        "/api/themes/neon/preview",
+    ] {
+        let req = Request::builder()
+            .uri(uri)
+            .body(Body::empty())
+            .expect("build request");
+        let resp = app.clone().oneshot(req).await.expect("oneshot");
+        assert_eq!(
+            resp.status(),
+            StatusCode::NOT_FOUND,
+            "expected 404 for {uri}"
+        );
+    }
+}