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

app/api/account/stalwart/jmap/route.ts

@@ -0,0 +1,46 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { getStalwartCredentials } from '@/lib/stalwart/credentials';
+
+/**
+ * POST /api/account/stalwart/jmap
+ *
+ * Passthrough to Stalwart's JMAP endpoint using the stored basic-auth
+ * context so the browser does not need access to the user's credentials.
+ *
+ * Body: standard JMAP request `{ using: string[], methodCalls: [...] }`
+ *
+ * In Stalwart 0.16 all management operations (password change, app
+ * passwords, API keys, account settings, etc.) are exposed as JMAP
+ * methods under the `x:` namespace on the same endpoint.
+ */
+export async function POST(request: NextRequest) {
+  try {
+    const creds = await getStalwartCredentials(request);
+    if (!creds) {
+      return NextResponse.json({ error: 'Not authenticated' }, { status: 401 });
+    }
+
+    const body = await request.text();
+
+    const response = await fetch(`${creds.serverUrl}/jmap/`, {
+      method: 'POST',
+      headers: {
+        'Authorization': creds.authHeader,
+        'Content-Type': 'application/json',
+      },
+      body,
+    });
+
+    const responseText = await response.text();
+    return new NextResponse(responseText, {
+      status: response.status,
+      headers: { 'Content-Type': response.headers.get('Content-Type') || 'application/json' },
+    });
+  } catch (error) {
+    logger.error('Stalwart JMAP passthrough error', {
+      error: error instanceof Error ? error.message : 'Unknown',
+    });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}