feat: inline search filters, folder icon picker, richer demo data & UI polish

- Replace advanced search panel with inline filter chips and collapsible fields
  (from/to/subject/body inputs, folder dropdown, attachment/flagged/read toggles)
- Add debounced auto-search on filter field changes
- Folder settings: add icon picker for custom per-folder icons, toast feedback,
  reorder sections (folder list first, roles second), empty-state illustration
- Avatar: show sender favicon for company domains, randomuser.me portraits for
  personal emails, custom avatars for demo senders, fallback to initials
- Add /api/favicon proxy route for fetching domain favicons
- Settings store: add senderFavicons toggle and folderIcons persistence
- Richer mock inbox with 11 realistic emails (GitHub, Slack, Stripe, Vercel, etc.)
- Navigation rail & sidebar cleanup, permanent-delete warning i18n (all 8 locales)
- Rename search toggle label to 'More', add body/folder filter translations

app/[locale]/settings/page.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState } from 'react';
+import { useState, useEffect } from 'react';
 import { useRouter } from '@/i18n/navigation';
 import { useTranslations } from 'next-intl';
 import { ArrowLeft, Settings as SettingsIcon } from 'lucide-react';
@@ -23,9 +23,19 @@ type Tab = 'appearance' | 'email' | 'account' | 'identities' | 'vacation' | 'cal
 export default function SettingsPage() {
   const router = useRouter();
   const t = useTranslations('settings');
-  const { client } = useAuthStore();
+  const { client, isAuthenticated } = useAuthStore();
   const [activeTab, setActiveTab] = useState<Tab>('appearance');
 
+  useEffect(() => {
+    if (!isAuthenticated) {
+      router.push('/login');
+    }
+  }, [isAuthenticated, router]);
+
+  if (!isAuthenticated) {
+    return null;
+  }
+
   const supportsVacation = client?.supportsVacationResponse() ?? false;
   const supportsCalendar = client?.supportsCalendars() ?? false;
   const supportsSieve = client?.supportsSieve() ?? false;