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.