feat: migrate Stalwart management API to JMAP x: methods (0.16)

Drops the 0.15 REST management API and routes all account/auth/crypto/
principal operations through Stalwart 0.16's schema-driven JMAP
endpoint via a single passthrough (/api/account/stalwart/jmap).

- New client helper `stalwartJmap` + typed `requireResult`
- account-security-store rewritten against x:AccountPassword, x:AppPassword,
  x:AccountSettings, x:Account (with currentSecret for TOTP ops)
- Client-side TOTP setup via `otpauth`; server-generated app password
  secrets shown once on create
- Admin check switched to /api/account permissions
  (sysAccountQuery/sysTenantQuery/sysSystemSettingsGet)
- Removed sieve vacation-overwrite workaround (fixed upstream #1251)
- Deleted old REST routes, StalwartClient, stale tests; added new
  tests for passthrough + store

components/layout/navigation-rail.tsx

@@ -223,11 +223,12 @@ export function NavigationRail({
     let cancelled = false;
     const headers = getActiveAccountSlotHeaders();
     if (!headers['X-JMAP-Cookie-Slot']) return;
-    apiFetch('/api/admin/stalwart-check', { headers })
+    apiFetch('/api/admin/auth', { headers })
       .then(res => res.json())
       .then(data => {
-        if (!cancelled && data.isStalwartAdmin) {
-          setIsStalwartAdmin(true);
+        if (cancelled || !data.stalwartAdmin) return;
+        setIsStalwartAdmin(true);
+        if (!data.authenticated) {
           // Pre-create admin session so /admin works even after full page navigation
           apiFetch('/api/admin/auth', {
             method: 'POST',