Enable edge-to-edge message list with navigation bar scrim behavior

[Gael43]

Summary

This PR adds edge-to-edge rendering for the message list with a transparent system navigation bar and a bottom scrim behavior adapted to the active navigation mode.

Changes

• Updated shared Theme2 light/dark themes:
  • android:navigationBarColor is transparent
  • android:windowLightNavigationBar is set per theme
• Updated BaseActivity:
  • Configures both isAppearanceLightStatusBars and isAppearanceLightNavigationBars
  • Adds protected inset extension points:
    • getContentContainerBottomInset(systemBarsBottom, imeBottom)
    • shouldConsumeRootInsets()
• Updated MessageHomeActivity:
  • Uses imeBottom for content bottom inset
  • Returns root insets to child views (shouldConsumeRootInsets() = false)
• Updated message list layout:
  • Added @id/message_list_navigation_bar_scrim as a bottom overlay scrim
• Updated BaseMessageListFragment:
  • Applies dynamic system-bar padding to the RecyclerView (left/right/bottom)
  • Sizes/positions the bottom scrim from navigation bar insets
  • Detects navigation mode from WindowInsets
  • Gesture navigation: scrim is visible by default, hides on scroll down, reappears on scroll up and at top
  • 3-button navigation: scrim remains visible at all times (opaque bottom area)
  • FAB insets handling uses a single WindowInsets listener with consistent system-bar-aware margins
• Updated MessageViewFragment:
  • Applies dynamic bottom padding from system bar insets to prevent overlap of bottom actions/content

Manual Validation

• Light theme: transparent nav bar with readable dark nav icons
• Dark theme: transparent nav bar with readable light nav icons
• Gesture navigation:
  • List content scrolls under the bottom system area
  • Scrim visibility follows scroll direction
  • Last list item remains reachable
• 3-button navigation:
  • Bottom area remains opaque at all times
  • No transparency effect while scrolling
  • FAB remains correctly positioned relative to nav controls
• Message view:
  • Bottom actions/content are not obscured by the navigation area