feat(plugins): add plugin windows, CLI actions, and data dirs

- Add `--plugin-action <id>/<name>` CLI flag, dispatched via runPluginAction
- Add ctx.ui.openWindow for plugin-owned BrowserWindows with sandboxed preload
- Add ctx.dataDir (`plugin-data/<id>/`), ctx.actions.run, ctx.mpv.ensureRunning
- Emit mpv end-file events to plugins with reason, entry id, and error
- Run Windows .cmd/.bat shims through cmd.exe in exec, killing the tree on abort
- Update plugin docs, usage, architecture notes, and e2e coverage

docs/architecture/domains.md

@@ -34,7 +34,7 @@ Read when: you need to find the owner module for a behavior or test surface
 - AniList tracking + character dictionary: `src/core/services/anilist/`, `src/main/runtime/composers/anilist-*`, `src/main/character-dictionary-runtime.ts`, `src/main/character-dictionary-runtime/`
 - TMDB live-action metadata: `src/core/services/tmdb/` (client + exact-title resolver), `src/core/services/immersion-tracker/live-action-link.ts` (links an entry to a TMDB title and merges other holders of the same title). The AniList cover-art fetcher calls the resolver as its fallback; `imm_anime.media_kind` marks the result and keeps the entry out of AniList season repair.
 - Jellyfin integration: `src/core/services/jellyfin*.ts`, `src/main/runtime/composers/jellyfin-*`
-- User plugins: `src/core/services/plugins/` (host, `ctx`, `exec`), `src/main/runtime/plugin-panel-runtime.ts` (panel modal), `src/renderer/modals/plugin-panel*.ts`; example in `assets/plugins/`
+- User plugins: `src/core/services/plugins/` (host, `ctx`, `exec`), `src/main/runtime/plugin-panel-runtime.ts` (panel modal), `src/main/runtime/plugin-window-runtime.ts` (plugin windows), `src/renderer/modals/plugin-panel*.ts`; example in `assets/plugins/`
 - Window trackers: `src/window-trackers/`
 - Stats HTTP app: `src/core/services/stats-server.ts`, with route groups and shared route support
   in `src/core/services/stats-server/`