feat(config): show default keybindings in generated example config (#64)

- Expand `keybindings` array in `config.example.jsonc` to list all built-in defaults instead of `[]`
- Inject `DEFAULT_KEYBINDINGS` into template when resolved config has an empty keybindings array
- Add regression tests for template parity, default binding compile/action mapping, overlay keyboard dispatch, and mpv plugin registration/dispatch
- Add fullscreen binding to docs shortcut tables and clarify keybindings can target mpv commands or session actions

docs-site/shortcuts.md

@@ -38,6 +38,7 @@ These control playback and subtitle display. They require overlay window focus.
 | Shortcut             | Action                                              |
 | -------------------- | --------------------------------------------------- |
 | `Space`              | Toggle mpv pause                                    |
+| `F`                  | Toggle fullscreen                                   |
 | `V`                  | Toggle primary subtitle bar visibility              |
 | `J`                  | Cycle primary subtitle track                        |
 | `Shift+J`            | Cycle secondary subtitle track                      |