feat(fullscreen): implement dynamic playbar resizing and update layout handling

[LargeModGames]

This pull request updates how the fullscreen lyrics and cover art views handle the playbar, making its height fully configurable and allowing it to be hidden or resized with existing keybindings. The implementation now consistently honors the user's playbar height setting across fullscreen views, improves the centering of cover art when the playbar is hidden, and adds comprehensive tests for these behaviors.

Fullscreen playbar resizing and layout improvements:

• Added a new fullscreen_view_layout function in core/layout.rs to calculate the content and playbar areas in fullscreen views based on the configured playbar height. If the playbar height is set to zero, the playbar is hidden and the content area fills the screen. Includes new tests for these behaviors. [1] [2]
• Updated fullscreen lyrics and cover art rendering (draw_lyrics_view, draw_cover_art_view in ui/player.rs) to use the new layout logic, so the playbar area is only rendered if its height is nonzero.
• Refactored mouse event handling to use the new layout function for hit-testing the playbar in fullscreen views, and added tests to ensure correct behavior when the playbar is resized or hidden. [1] [2]

Cover art centering and sizing fixes:

• Improved cover art centering logic in fullscreen mode: the image is now always centered within the available area, even when the playbar is hidden. The image size is calculated using a new method that accounts for the actual rendered size. [1] [2] [3]
• Fixed an off-center rendering bug when the playbar is hidden in fullscreen cover art view.

Code cleanup and documentation:

• Removed the hardcoded FULLSCREEN_VIEW_PLAYBAR_HEIGHT constant in favor of using the user-configured value throughout the codebase. [1] [2]
• Updated the changelog to document the new fullscreen playbar resizing feature and the cover art centering fix.

[LargeModGames]

closes #208