fix: allow back button to minimize app from FEED

[barrydeen]

Summary

• Users have complained that pressing the system back button on the home screen does nothing. The BackHandler in Navigation.kt was consuming the press on FEED to keep the app open.
• Disable the handler on FEED so Android's default behavior (minimize to background) runs. Non-FEED app routes still pop the back stack, falling back to FEED when empty.

Test plan

• On FEED, press the system back button — app minimizes to background (task remains in recents)
• From a non-FEED screen (e.g. PROFILE_EDIT, DM_CONVERSATION), back pops to the previous screen as before
• From a deep-linked non-FEED screen with empty back stack, back navigates to FEED