fix: prevent back button from closing the app

[barrydeen]

Summary

• Add a global BackHandler in Navigation.kt that prevents the system back button from ever exiting the app
• On the feed screen, back press is consumed (no-op since there's nowhere to go)
• On any other app screen, pops the back stack normally; if the stack is empty, navigates to the feed as a fallback

Test plan

• On the feed screen, press the Android back button — app should stay on feed
• Navigate to a nested screen (e.g. thread, profile), press back — should pop back normally
• Deep-link or navigate to a screen where the back stack is empty, press back — should land on feed instead of closing