Show 'Go to App' button for authenticated users on landing page

Instead of displaying sign-in buttons to already authenticated users,
the homepage now detects authentication status and shows a 'Go to App'
button that redirects to the home page.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

messages/en.json

@@ -23,6 +23,7 @@
       "title-3": "How much are my expenses?",
       "text-1": "These are the questions this simple and free app will help you with ...",
       "text-2": "... try it & use it completly free!",
+      "go-to-app": "Go to the App",
       "in-app-browser-warning": "<strong>WARNING:</strong> we detected you're using an in-app browser.<br></br>This might prevent this app to work properly.<hint>Please use a regular browser to sign in (rezije.app) 😉</hint>",
       "video-url": "/welcome-demo-vp9-25fps-1500bps.webm",
       "image-url": "/hero.png",