fix: fullscreen video not adapting to landscape orientation

[barrydeen]

Summary

• Replace Compose Dialog with native Android Dialog using Theme_Black_NoTitleBar_Fullscreen for the fullscreen video player
• Compose's Dialog creates a separate window that doesn't resize properly on orientation change when the Activity handles configChanges, causing the video to be pushed out of view and shrunk in landscape mode
• Native Android Dialog with a fullscreen theme handles orientation changes at the platform level, so the PlayerView resizes naturally when the screen rotates

Test plan

• Fullscreen video plays correctly in portrait
• Rotate portrait → landscape while fullscreen — video adjusts to fill screen
• Open fullscreen while already in landscape — video fills screen correctly
• Rotate landscape → portrait — video adjusts back
• Back button/gesture dismisses fullscreen
• Close button dismisses fullscreen
• Download button works from fullscreen
• Copy URL button copies to clipboard
• System bars hidden during fullscreen, restored after dismiss
• Fullscreen multiple different videos sequentially — no leaked players or audio
• Playback resumes from inline position when entering fullscreen