Remove fullscreen mode on mobile home screen

[Himanth-reddy]

Summary

Refactor the mobile UI to remove forced fullscreen mode on non-player screens. All home, browse, and profile screens now display standard transparent status and navigation bars, while the video player retains its immersive edge-to-edge behavior.

Problem

The app was using Theme.Leanback as a parent for all devices, which forces android:windowFullscreen=true. This resulted in a sub-optimal mobile experience where system bars were permanently hidden or poorly integrated, making the app feel "trapped" and inconsistent with standard Android UX.

Solution

1. Conditional Theming: Introduced Theme.ArflixTV.Mobile which overrides the fullscreen flag.
2. Edge-to-Edge with Insets: Switched the root layout to use WindowCompat.setDecorFitsSystemWindows(window, false) (Edge-to-Edge) globally.
3. Manual Inset Handling: Used Compose systemBarsPadding() on the root Column for mobile only. This ensures UI content stays within the safe area while the background gradient bleeds seamlessly behind the transparent bars.
4. Dynamic Fullscreen: Added a DisposableEffect in PlayerScreen to hide system bars only during playback on mobile.
5. Aesthetics: Configured transparent bar colors with white (light) icons to match the dark Arctic Fuse theme.

Changes

app/src/main/res/values/themes.xml

• Added Theme.ArflixTV.Mobile inheriting from the main theme but with windowFullscreen=false and transparent bar colors.
• Updated Theme.ArflixTV.Splash to use the mobile theme as postSplashScreenTheme.

app/src/main/kotlin/com/arflix/tv/MainActivity.kt

• Implemented logic to override the splash theme back to the Leanback fullscreen theme only on TV devices.
• Configured window flags for transparent status/navigation bars and white icon appearance.
• Re-ordered root modifiers: .background() -> .systemBarsPadding() to allow the background to fill the status/nav bar areas.

app/src/main/kotlin/com/arflix/tv/ui/screens/player/PlayerScreen.kt

• Added DisposableEffect to manage immersive mode specifically for the mobile video player, ensuring it returns to normal once dismissed.

Verification

• Home screen shows status and navigation bars on mobile.
• Navigation bar is transparent and shows app background.
• Content is correctly offset by systemBarsPadding.
• Video player still enters immersive fullscreen on mobile.
• TV behavior remains immersive fullscreen globally.