[P2] Single Activity + type-safe Navigation Compose + session-aware start

[amaury901130]

One MainActivity hosting a NavHost with @serializable routes, nested auth/main graphs, popUpTo on sign in/out. Start destination based on session state (fixes signed-in users landing on Sign Up).

Acceptance criteria

• One Activity
• Signed-in user opens directly to Profile

Reference: Android Base — Modernization Audit & Recommendations — section: Jetpack Compose migration