feat(admin): add asset picker with favorites, drag-and-drop reordering, preview links, and keyboard shortcuts

- Asset Picker: Visual browser for hero images with Favorites tab (shown first)
  and All Photos tab. Includes infinite scroll and UUID fallback input for
  assets not easily found in the grid.
- Drag & Drop: All lists (hero images, standalone albums, subpage albums,
  subpage tiles) are now reorderable via drag handles using @dnd-kit.
- Preview Links: 'Preview Site' button in save bar, '↗ Site' in header, and
  per-subpage preview links with URL slug preview shown inline.
- Keyboard Shortcuts: ⌘+S / Ctrl+S to save in both Pages and Settings tabs.
- Unsaved Changes Guard: beforeunload warning prevents accidental data loss.
- New API route /api/admin/assets for browsing Immich assets with pagination
  and favorites filter via search/metadata endpoint.

app/admin/components/SettingsEditor.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 
 interface Settings {
   title?: string;
@@ -67,6 +67,37 @@ export default function SettingsEditor() {
     loadSettings();
   }, []);
 
+  // ── Keyboard shortcut: ⌘+S / Ctrl+S ─────────────────────────
+  const handleSaveRef = useCallback(() => {
+    if (dirty && !saving) {
+      handleSave();
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [dirty, saving, settings]);
+
+  useEffect(() => {
+    function handleKeyDown(e: KeyboardEvent) {
+      if ((e.metaKey || e.ctrlKey) && e.key === 's') {
+        e.preventDefault();
+        handleSaveRef();
+      }
+    }
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [handleSaveRef]);
+
+  // ── Unsaved changes guard ────────────────────────────────────
+  useEffect(() => {
+    function handleBeforeUnload(e: BeforeUnloadEvent) {
+      if (dirty) {
+        e.preventDefault();
+        e.returnValue = '';
+      }
+    }
+    window.addEventListener('beforeunload', handleBeforeUnload);
+    return () => window.removeEventListener('beforeunload', handleBeforeUnload);
+  }, [dirty]);
+
   async function loadSettings() {
     setLoading(true);
     try {