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/routes/settings/Settings.tsx

@@ -8,6 +8,7 @@ import { useT } from "../../i18n";
 import { api, ApiError } from "../../lib/api";
 import { useMe } from "../../lib/auth";
 import { useTheme } from "../../lib/theme";
+import { PluginSlot } from "../../plugins/PluginSlot";
 
 const SECTIONS = ["profile", "security", "accounts", "appearance"] as const;
 type Section = (typeof SECTIONS)[number];
@@ -44,6 +45,13 @@ export function Settings() {
           {section === "security" && <SecuritySection />}
           {section === "accounts" && <AccountsSection />}
           {section === "appearance" && <AppearanceSection />}
+          {/* Plugin-contributed settings UI (SPEC §11.3 settings.page), under Accounts.
+              Matches the accounts section width (max-w-5xl) with a gap above it. */}
+          {section === "accounts" && (
+            <div className="mt-8 max-w-5xl">
+              <PluginSlot name="settings.page" />
+            </div>
+          )}
         </main>
       </div>
     </div>