feat: add visual admin panel with page builder and settings editor

- Add /admin route with password-protected visual page builder
- Page Builder: manage hero images, standalone albums, subpages, and sections
- Album Picker: browse all shared Immich albums with search and filtering
- Settings Editor: theme, grid, footer, legal, SEO configuration
- YAML service with atomic writes and automatic backups (content/.backups/)
- HMAC-signed admin sessions (24h expiry, HttpOnly cookies)
- API endpoints: /api/admin/{auth,gallery,settings,albums,reload}
- Enable with ADMIN_PASSWORD env var; disabled by default
- invalidateConfigCache() export for hot-reload after saves

app/admin/components/AlbumPicker.tsx

@@ -0,0 +1,97 @@
+'use client';
+
+import { useState, useMemo } from 'react';
+
+interface ImmichAlbumInfo {
+  id: string;
+  albumName: string;
+  description: string;
+  thumbnailAssetId: string | null;
+  assetCount: number;
+  isConfigured: boolean;
+}
+
+interface Props {
+  albums: ImmichAlbumInfo[];
+  onSelect: (albumId: string) => void;
+  onClose: () => void;
+  usedAlbumIds: Set<string>;
+}
+
+export default function AlbumPicker({ albums, onSelect, onClose, usedAlbumIds }: Props) {
+  const [search, setSearch] = useState('');
+
+  const filtered = useMemo(() => {
+    if (!search.trim()) return albums;
+    const q = search.toLowerCase();
+    return albums.filter(
+      (a) =>
+        a.albumName.toLowerCase().includes(q) ||
+        a.description.toLowerCase().includes(q) ||
+        a.id.includes(q),
+    );
+  }, [albums, search]);
+
+  return (
+    <div className="picker-overlay" onClick={onClose}>
+      <div className="picker-modal" onClick={(e) => e.stopPropagation()}>
+        <div className="picker-header">
+          <h3>Select Album</h3>
+          <button className="admin-btn-icon" onClick={onClose}>
+            ×
+          </button>
+        </div>
+
+        <div className="picker-search">
+          <input
+            type="text"
+            placeholder="Search albums by name or UUID..."
+            value={search}
+            onChange={(e) => setSearch(e.target.value)}
+            autoFocus
+          />
+        </div>
+
+        <div className="picker-list">
+          {filtered.length === 0 && (
+            <p className="empty-hint">
+              No albums found. Make sure you have shared albums in Immich.
+            </p>
+          )}
+          {filtered.map((album) => {
+            const isUsed = usedAlbumIds.has(album.id);
+            return (
+              <div
+                key={album.id}
+                className={`picker-item ${isUsed ? 'used' : ''}`}
+                onClick={() => !isUsed && onSelect(album.id)}
+              >
+                <div className="picker-item-thumb">
+                  {album.thumbnailAssetId ? (
+                    <div className="picker-thumb-placeholder">📷</div>
+                  ) : (
+                    <div className="picker-thumb-placeholder">📁</div>
+                  )}
+                </div>
+                <div className="picker-item-info">
+                  <span className="picker-item-name">{album.albumName}</span>
+                  <span className="picker-item-meta">
+                    {album.assetCount} photos
+                    {album.description && ` · ${album.description.slice(0, 40)}`}
+                  </span>
+                </div>
+                <div className="picker-item-action">
+                  {isUsed ? (
+                    <span className="picker-used-badge">In use</span>
+                  ) : (
+                    <span className="picker-select-badge">Select</span>
+                  )}
+                </div>
+              </div>
+            );
+          })}
+        </div>
+      </div>
+    </div>
+  );
+}