M3 plugin platform: frontend host, reference plugins, UI kit + docs

Completes M3 (SPEC §11 + §12 acceptance) on top of the backend platform (4e077f1).

Frontend host (§11.3):
- src/plugins/: manifest loader, runtime ESM bundle loader (framework-agnostic
  mount contract), PluginSlot, PluginPage (app.page at /p/*), plugin context
  (scoped api/events/i18n), capabilities hook. Wired slots into ThreadView
  (message.actions/message.banner), MailLayout sidebar (sidebar.section + page nav)
  and Settings→Accounts (settings.page); ws.ts fans out core + plugin.* events.
- plugin-ui.css: theme-aware .wm-* classes so bundles don't re-derive inline styles.

SDK additions:
- CurrentUserId + require_csrf (plugin-router auth), messages.envelope/header,
  actions.set_flags/trash, sdk.enqueue.

Reference plugins (plugins/, installed via webmail.plugins entry points):
- hello: skeleton page-plugin (app.page + plg_hello_note w/ RLS + cron job) —
  install→run→uninstall --drop-data leaves zero residue.
- labels: message.actions + sidebar.section slots, KV only, zero core diffs.
- filters: message.new hook -> match (subject/from/to) -> Mark read / Pin / Delete
  (delete = move thread to Trash via the core path).

docs/plugins.md: full authoring guide + a complete "Unsubscribe button" worked
example (the §12 "writable from docs alone" bar).

Also: conftest resets the arq pool between tests (loop-bound global, like engine/redis).

Gates: backend ruff + mypy --strict (107) + full suite 337; frontend tsc + eslint
+ vitest (11) + vite build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/tests/conftest.py

@@ -43,10 +43,14 @@ async def _reset_global_engine() -> AsyncIterator[None]:
     await get_redis().flushdb()
     yield
     from app.db import dispose_engine
+    from app.queue import close_arq_pool
     from app.redis_client import close_redis
 
     await dispose_engine()
     await close_redis()
+    # The arq enqueue pool is a loop-bound global like the engine/redis; reset it so
+    # a test that enqueues doesn't reuse a pool from a previous (closed) event loop.
+    await close_arq_pool()
 
 
 @pytest_asyncio.fixture