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-site/usage.md

@@ -104,6 +104,7 @@ subminer doctor                    # check dependencies, config, and the mpv soc
 subminer generate-subs video.mkv   # make Japanese subtitles from the audio
 subminer logs -e                   # export a log ZIP for bug reports
 subminer app --setup               # reopen first-run setup
+subminer app --plugin-action <id>/<name>  # run a plugin action
 subminer -u                        # update SubMiner
 ```