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/architecture/README.md

@@ -37,6 +37,7 @@ Update checks and startup launcher migration share a serialized update-state sto
 - `subtitle-generation-reference.ts` ranks mpv's loaded text subtitle tracks, excludes signs/songs and forced references, and extracts timing hints with FFmpeg. The overlay and launcher snapshot references only for matching media, including active subtitle delays. Hints guide long-passage cuts with or without VAD; they never limit audio coverage or replace Whisper timestamps.
 - `src/main/runtime/subtitle-selection.ts` reads and validates mpv subtitle tracks and applies primary/secondary selections. Its opt-in session shortcut opens the shared overlay modal window, with renderer focus and subtitle suppression handled by the modal registry.
 - `src/shared/session-key-sequences.ts` rejects sequence prefixes reserved by single-key actions. The session-binding compiler reserves configured and built-in overlay keys; `src/main/runtime/session-bindings-runtime.ts` adds active mpv bindings and publishes the effective list to both the plugin artifact and the renderer through `session-bindings:changed`. mpv no-op `ignore` bindings do not reserve prefixes.
+- `src/core/services/plugins/` owns user plugins: discovery from `<config dir>/plugins/<id>/plugin.json` for ids enabled in `plugins`, the per-plugin `ctx`, guarded callbacks, and `exec` (no shell, AppImage `LD_LIBRARY_PATH` dropped). `main.ts` supplies the host's deps, attaches each new mpv client so plugin listeners survive client replacement, starts plugins from the app-ready path after logging is configured, and unloads them on `before-quit`. Plugins run in the main process with no sandbox. Plugin actions are the `runPluginAction` session action (keybinding command `__plugin-action:<id>/<name>`). `src/main/runtime/plugin-panel-runtime.ts` owns the single `plugin-panel` modal: a new panel aborts the previous handle's signal, updates broadcast to both overlay windows keyed by `panelId`, and the latest content is re-sent once the renderer acknowledges the open.
 - `src/renderer/` owns overlay rendering and input behavior.
 - `src/config/` owns config definitions, defaults, loading, and resolution.
 - `src/types/` owns shared cross-runtime contracts via domain entrypoints; `src/types.ts` stays a compatibility barrel.