Polishes the loading placeholder in the full player and refactors the `FileExplorerBottomSheet` to use a standard `CenterAlignedTopAppBar`.

**`FullPlayerContent.kt`:**
- Adjusts spacing, sizing, and corner radius of various placeholder elements for a more refined look.
- Modifies the placeholder icon's tint color and removes the alpha from the background surface.
- Increases the minimum height and spacing in the metadata and controls sections.

**`FileExplorerBottomSheet.kt`:**
- Replaces the custom header `Row` with a standard `CenterAlignedTopAppBar`.
- This change centralizes the title and moves the close and refresh buttons into the standard `navigationIcon` and `actions` slots.

**Other Changes:**
- **`SettingsScreen.kt`**: Updates the "Experimental" settings icon from a warning symbol to a science beaker.
- **`ExperimentalSettingsScreen.kt`**: Reduces padding and spacing for a more compact layout.