Polish Android app: auth gate, 3-tab layout, real Google Sign-In

- Add google_sign_in dependency
- AuthWrapper routes to SignInPage (with Memory by Clouderized branding)
  or MainScaffold based on stored session
- MainScaffold: Gallery / Upload / Settings tabs — settings inline card removed
- Settings tab: agent status, LAN pairing token, sign-out with confirmation
- FullPhotoPage title shows filename instead of internal path label
- App bar title changed to MemoriC
- .env holds GOOGLE_SERVER_CLIENT_ID for dart-define-from-file builds
- .gitignore excludes memoric-app/.env

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

.gitignore

@@ -27,3 +27,6 @@ agent/photos/
 
 # OS
 .DS_Store
+
+# Local secrets
+memoric-app/.env