feat(plugins): add JavaScript plugin system with overlay panel

- Load plugins from <config dir>/plugins/<id>/ when enabled under `plugins` in config
- Expose ctx with mpv events/commands, timers, subtitle context, actions, notify, exec, and panel
- Bind plugin actions via `__plugin-action:<id>/<action>` keybindings (runPluginAction session action)
- Add plugin-panel modal with markdown rendering; closing aborts the panel signal
- Add ai-explain example plugin that queries `codex exec` for the current line
- Add e2e harness `files` option and plugin e2e scenario; document plugins in docs-site and internal docs

docs-site/.vitepress/config.ts

@@ -221,6 +221,7 @@ const sidebar: DefaultTheme.SidebarItem[] = [
       { text: 'AniList', link: '/anilist-integration' },
       { text: 'AniSkip', link: '/aniskip-integration' },
       { text: 'Character Dictionary', link: '/character-dictionary' },
+      { text: 'Plugins', link: '/plugins' },
     ],
   },
   {