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/components/thread/ThreadView.tsx

@@ -5,6 +5,7 @@ import { useT } from "../../i18n";
 import { api } from "../../lib/api";
 import { printMessages } from "../../lib/print";
 import type { AccountOut, MailboxOut, MessageEnvelope } from "../../lib/types";
+import { PluginSlot } from "../../plugins/PluginSlot";
 import { Composer, type ComposerInit } from "../compose/Composer";
 import { Icon } from "../Icon";
 import { MessageBody } from "./MessageBody";
@@ -357,6 +358,8 @@ function MessageItem({
                     onDelete={onDelete}
                     replyDisabled={!canReply}
                   />
+                  {/* Plugin-contributed per-message actions (SPEC §11.3 message.actions). */}
+                  <PluginSlot name="message.actions" props={{ messageId: message.id }} />
                 </div>
               </div>
               {alsoIn.length > 0 && (
@@ -386,6 +389,8 @@ function MessageItem({
               )}
             </div>
           </div>
+          {/* Plugin-contributed per-message banner (SPEC §11.3 message.banner). */}
+          <PluginSlot name="message.banner" props={{ messageId: message.id }} />
           <div className="px-4 pb-3">
             <MessageBody message={message} onMailto={onMailto} />
           </div>