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/hooks/useProfile.ts

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { fetchProfile } from "../lib/nostr";
+import { dbLoadProfile, dbSaveProfile } from "../lib/db";
 
 const profileCache = new Map<string, any>();
 const pendingRequests = new Map<string, Promise<any>>();
@@ -18,20 +19,38 @@ export function useProfile(pubkey: string) {
       return;
     }
 
-    // Deduplicate requests for the same pubkey
+    // Kick off relay fetch (deduplicated across simultaneous callers)
     if (!pendingRequests.has(pubkey)) {
-      const request = fetchProfile(pubkey).then((p) => {
-        profileCache.set(pubkey, p ?? null);
-        pendingRequests.delete(pubkey);
-        return p;
-      }).catch(() => {
-        pendingRequests.delete(pubkey);
-        return null;
-      });
+      const request = fetchProfile(pubkey)
+        .then((p) => {
+          const result = p ?? null;
+          profileCache.set(pubkey, result);
+          pendingRequests.delete(pubkey);
+          if (result) dbSaveProfile(pubkey, JSON.stringify(result));
+          return result;
+        })
+        .catch(() => {
+          pendingRequests.delete(pubkey);
+          return null;
+        });
       pendingRequests.set(pubkey, request);
     }
 
+    // Show SQLite cached profile immediately while the relay request is in-flight.
+    // `settled` prevents the stale cached value from overwriting a fresh relay result.
+    let settled = false;
+    dbLoadProfile(pubkey).then((cached) => {
+      if (!settled && cached && !profileCache.has(pubkey)) {
+        try {
+          setProfile(JSON.parse(cached));
+        } catch {
+          // Corrupt cache entry — ignore
+        }
+      }
+    });
+
     pendingRequests.get(pubkey)!.then((p) => {
+      settled = true;
       setProfile(p ?? null);
     });
   }, [pubkey]);