Simple Quick Keys

### Added
- The panel is now a 16:9 rectangle with cut corners and a cleaner frame.
- Slots are semi-transparent, so the panel background or your own image shows through.
- **Custom panel background colour**: adjust the whole panel's colour (R/G/B + opacity) in edit mode.
- **Custom background image**: drop your own image into `config/key_panel/backgrounds/` and pick it from the thumbnail grid in the "Image" screen. png / jpg are supported; 16:9 images (e.g. 1920x1080) are recommended and are cropped to fill without stretching. "Open folder" and "Refresh" buttons are built in, so no game restart is needed after adding an image.
- Slot colours now support opacity: the "Slots" target changes the colour and opacity of all slots at once.
- All interface text is now translatable through language files / resource packs.

### Improved
- Edit mode was re-laid out: "Slots / Background / Image" buttons on the left, R / G / B / A sliders and a square colour preview on the right. Click the preview to reset to the default colour.
- The background image picker is now a thumbnail grid, so you can see each image before choosing it.
- Panels and controls share a consistent semi-transparent look; no more stray colour blocks in the corners.

### Fixed
- Fixed slot frames occasionally not drawing completely.
- Fixed opacity having no effect, sliders not following the mouse, and the value jumping at either end.
- Fixed stray colour blocks in the panel corners and the frame being covered by the background.
- Fixed some interface text overlapping the frame decoration line.

### Notes
- Client-side only; nothing needs to be installed on the server.
- Config files are created under `config/key_panel/` on first launch; colours from older versions are migrated automatically.