Mobile: bottom tab bar (Cards · Collectibles · Agents · Settings)

Restructure the app into an Expo Router (tabs) group with a bottom navbar:
- 🎒 Cards (bag-personal) = the gallery (moved from app/index.tsx).
- 💰 Collectibles (sack) + 🤖 Agents (robot) = new empty-state screens.
- ⚙️ Settings (cog) = demo settings (app info + Clear all cards).
Root stack hides the (tabs) header; removed the Phase-1 (tabs)/cards.tsx and
pointed the dormant (auth)/login redirect at "/". localCardsStore gains clear().

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

cardclaws-mobile/app/(auth)/login.tsx

@@ -37,7 +37,7 @@ export default function LoginScreen() {
       } else {
         await login(email, password);
       }
-      router.replace("/(tabs)/cards");
+      router.replace("/");
     } catch {
       setError(mode === "login" ? "Invalid email or password" : "Could not create account");
     } finally {