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

frontend/src/lib/ws.ts

@@ -22,6 +22,19 @@ const ALLOWED_EVENTS = new Set([
   "outbox.state_changed",
 ]);
 
+// Lightweight in-process fan-out of WS frames to plugin subscribers (SPEC §11.3).
+// Plugins receive plugin.{name}.* events plus any core event they subscribe to.
+export type WsListener = (event: string, payload: Record<string, unknown>) => void;
+
+const wsListeners = new Set<WsListener>();
+
+export function subscribeWs(listener: WsListener): () => void {
+  wsListeners.add(listener);
+  return () => {
+    wsListeners.delete(listener);
+  };
+}
+
 export function connectWs(queryClient: QueryClient): () => void {
   const proto = window.location.protocol === "https:" ? "wss" : "ws";
   const socket = new WebSocket(`${proto}://${window.location.host}/api/ws`);
@@ -40,8 +53,21 @@ export function connectWs(queryClient: QueryClient): () => void {
       return;
     }
     const result = frameSchema.safeParse(parsed);
-    if (!result.success || !ALLOWED_EVENTS.has(result.data.event)) return;
+    if (!result.success) return;
     const { event: name, payload } = result.data;
+    const isCore = ALLOWED_EVENTS.has(name);
+    // Fan out core + plugin.* events to plugin subscribers (a failing listener
+    // must not break the others or the cache invalidation below).
+    if (isCore || name.startsWith("plugin.")) {
+      for (const listener of wsListeners) {
+        try {
+          listener(name, payload);
+        } catch (err) {
+          console.error("ws listener failed", err);
+        }
+      }
+    }
+    if (!isCore) return;
     // Coarse invalidation: refetch affected lists (SPEC §9.2 contract).
     if (name === "sync.progress" || name.startsWith("message.") || name === "thread.updated") {
       queryClient.invalidateQueries({ queryKey: ["threads"] });