Polish fullscreen to mini-player transition

[wizdom13]

• Cross-fade fullscreen controls and mini-player metadata with eased progress tied directly to the bottom-sheet drag
• Keep the current video frame visible while the player sheet is moving instead of showing the black surface cover during layout-only transitions
• Keep the loading spinner available while making its full black background transparent during the transition
• Delay restoring normal surface protection briefly after the sheet settles so the orientation/layout handoff can finish without a black flash
• Preserve the existing fullscreen orientation restore, playback state, position, and queue behavior
• Add regression coverage for continuous transition progress and visual-continuity wiring
• Relates to General UI/UX polish needed (missing animations, screen flickering) #368