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/tests/theme_store_round_trip.rs

@@ -0,0 +1,118 @@
+//! End-to-end theme-store test: a real `solitaire_server` router
+//! serving a scanned catalog over a localhost TCP socket, consumed by
+//! [`solitaire_data::ThemeStoreClient`].
+//!
+//! Mirrors the `sync_round_trip` harness: `TcpListener` on port 0,
+//! router in a background `tokio::spawn`, no explicit shutdown.
+
+#![cfg(not(target_arch = "wasm32"))]
+
+use std::io::Write as _;
+use std::path::{Path, PathBuf};
+
+use solitaire_data::ThemeStoreClient;
+use solitaire_server::theme_store::ThemeStore;
+
+/// Minimal theme archive: the catalog scan only reads the `meta`
+/// block, so no face SVGs are needed.
+fn write_store_zip(dir: &Path, file_name: &str, id: &str, name: &str) -> PathBuf {
+    let manifest = format!(
+        r#"(
+    meta: (
+        id: "{id}",
+        name: "{name}",
+        author: "Round Trip",
+        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
+}
+
+/// Spawn the test server with a theme store scanned from `store_dir`
+/// and return its base URL.
+async fn spawn_store_server(store_dir: &Path) -> String {
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0")
+        .await
+        .expect("failed to bind test listener");
+    let addr = listener.local_addr().expect("listener has no local addr");
+
+    let pool = solitaire_server::build_test_pool().await;
+    let app =
+        solitaire_server::build_test_router_with_theme_store(pool, ThemeStore::scan(store_dir));
+
+    tokio::spawn(async move {
+        if let Err(e) = axum::serve(listener, app).await {
+            eprintln!("test server crashed: {e}");
+        }
+    });
+
+    format!("http://{addr}")
+}
+
+/// Catalog fetch → download → verified bytes match the file the
+/// server scanned. This is the exact path the engine's store UI runs.
+#[tokio::test]
+async fn catalog_fetch_and_verified_download_round_trip() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let zip_path = write_store_zip(dir.path(), "neon.zip", "neon", "Neon");
+    let base_url = spawn_store_server(dir.path()).await;
+
+    let client = ThemeStoreClient::new(&base_url);
+    let catalog = client.fetch_catalog().await.expect("fetch catalog");
+    assert_eq!(catalog.len(), 1);
+    let entry = &catalog[0];
+    assert_eq!(entry.id, "neon");
+
+    let bytes = client.download_theme(entry).await.expect("download");
+    assert_eq!(bytes, std::fs::read(&zip_path).expect("read zip"));
+}
+
+/// A catalog entry whose hash no longer matches the served file must
+/// be rejected client-side — the importer never sees unverified bytes.
+#[tokio::test]
+async fn download_with_stale_catalog_hash_is_rejected() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    write_store_zip(dir.path(), "neon.zip", "neon", "Neon");
+    let base_url = spawn_store_server(dir.path()).await;
+
+    let client = ThemeStoreClient::new(&base_url);
+    let mut entry = client.fetch_catalog().await.expect("fetch catalog")[0].clone();
+    entry.sha256 = "0".repeat(64);
+
+    let err = client
+        .download_theme(&entry)
+        .await
+        .expect_err("mismatched hash must fail");
+    assert!(
+        matches!(
+            err,
+            solitaire_data::ThemeStoreError::ChecksumMismatch { .. }
+        ),
+        "expected ChecksumMismatch, got: {err:?}"
+    );
+}
+
+/// An empty (or absent) store directory serves an empty catalog — the
+/// client sees a store with nothing in it, not an error.
+#[tokio::test]
+async fn empty_store_yields_empty_catalog() {
+    let dir = tempfile::tempdir().expect("tempdir");
+    let base_url = spawn_store_server(dir.path()).await;
+
+    let client = ThemeStoreClient::new(&base_url);
+    let catalog = client.fetch_catalog().await.expect("fetch catalog");
+    assert!(catalog.is_empty());
+}