Fullscreen "Cinema mode" on YouTube and other quality-of-life improvements

A browser extension that makes YouTube's "Cinema mode" (Theater mode) truly occupy the full height of your browser window and fixes common focus issues with shortcut keys. ## 🚀 Features - **True Full-Height Theater Mode:** The player expands to `100vh`, giving you a "maximized" feel without entering OS-level fullscreen. - **Immersive UI:** The top header (masthead) automatically hides when in Theater mode and fades back in on hover. - **Smart Focus Management:** - Redirects arrow keys to the player if the mouse is hovering over it. - Automatically refocuses the player when interacting with progress or volume sliders. - **Enhanced Captions:** Improved subtitle readability with custom shadows and backgrounds. - **Custom Accents:** Features a sleek cyan playback progress bar. ## ⚙️ Configuration You can customize some styling variables in `extension/style_injection.css` or via the options page. By default, it uses a vibrant cyan color (`rgb(0, 239, 255)`) for the progress bar.