Add SQLite note and profile cache (roadmap #3)

- Rust: rusqlite (bundled) with WAL mode; wrystr.db in app data dir
  - db_save_notes: upsert batch of raw event JSON, prune to 500 kind-1 notes
  - db_load_feed: return N most-recent kind-1 raws for instant startup display
  - db_save_profile / db_load_profile: cache NDKUserProfile JSON by pubkey
  - Falls back to in-memory SQLite if the on-disk open fails
- src/lib/db.ts: typed invoke wrappers; all errors silenced (cache is best-effort)
- feed store: loadCachedFeed() populates notes before relay connects;
  loadFeed() merges fresh+cached (so relay returning fewer notes doesn't
  erase cached ones), then saves fresh notes to SQLite
- useProfile: reads SQLite cache to show avatar/name instantly while
  relay request is in-flight; saves result to SQLite after relay responds
- Feed: calls loadCachedFeed() first → notes visible before relay connects

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/feed/Feed.tsx

@@ -9,7 +9,7 @@ import { NDKEvent } from "@nostr-dev-kit/ndk";
 type FeedTab = "global" | "following";
 
 export function Feed() {
-  const { notes, loading, connected, error, connect, loadFeed } = useFeedStore();
+  const { notes, loading, connected, error, connect, loadCachedFeed, loadFeed } = useFeedStore();
   const { loggedIn, follows } = useUserStore();
 
   const [tab, setTab] = useState<FeedTab>("global");
@@ -17,6 +17,8 @@ export function Feed() {
   const [followLoading, setFollowLoading] = useState(false);
 
   useEffect(() => {
+    // Show cached notes immediately, then fetch fresh ones once connected
+    loadCachedFeed();
     connect().then(() => loadFeed());
   }, []);