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.

package-lock.json

@@ -9,6 +9,9 @@
       "version": "0.1.0",
       "license": "MIT",
       "dependencies": {
+        "@dnd-kit/core": "^6.3.1",
+        "@dnd-kit/sortable": "^10.0.0",
+        "@dnd-kit/utilities": "^3.2.2",
         "js-yaml": "^4.1.1",
         "leaflet": "^1.9.4",
         "next": "16.2.4",
@@ -283,6 +286,59 @@
         "node": ">=18"
       }
     },
+    "node_modules/@dnd-kit/accessibility": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/@dnd-kit/accessibility/-/accessibility-3.1.1.tgz",
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+      "license": "MIT",
+      "dependencies": {
+        "tslib": "^2.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=16.8.0"
+      }
+    },
+    "node_modules/@dnd-kit/core": {
+      "version": "6.3.1",
+      "resolved": "https://registry.npmjs.org/@dnd-kit/core/-/core-6.3.1.tgz",
+      "integrity": "sha512-xkGBRQQab4RLwgXxoqETICr6S5JlogafbhNsidmrkVv2YRs5MLwpjoF2qpiGjQt8S9AoxtIV603s0GIUpY5eYQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@dnd-kit/accessibility": "^3.1.1",
+        "@dnd-kit/utilities": "^3.2.2",
+        "tslib": "^2.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=16.8.0",
+        "react-dom": ">=16.8.0"
+      }
+    },
+    "node_modules/@dnd-kit/sortable": {
+      "version": "10.0.0",
+      "resolved": "https://registry.npmjs.org/@dnd-kit/sortable/-/sortable-10.0.0.tgz",
+      "integrity": "sha512-+xqhmIIzvAYMGfBYYnbKuNicfSsk4RksY2XdmJhT+HAC01nix6fHCztU68jooFiMUB01Ky3F0FyOvhG/BZrWkg==",
+      "license": "MIT",
+      "dependencies": {
+        "@dnd-kit/utilities": "^3.2.2",
+        "tslib": "^2.0.0"
+      },
+      "peerDependencies": {
+        "@dnd-kit/core": "^6.3.0",
+        "react": ">=16.8.0"
+      }
+    },
+    "node_modules/@dnd-kit/utilities": {
+      "version": "3.2.2",
+      "resolved": "https://registry.npmjs.org/@dnd-kit/utilities/-/utilities-3.2.2.tgz",
+      "integrity": "sha512-+MKAJEOfaBe5SmV6t34p80MMKhjvUz0vRrvVJbPT0WElzaOJ/1xs+D+KDv+tD/NE5ujfrChEcshd4fLn0wpiqg==",
+      "license": "MIT",
+      "dependencies": {
+        "tslib": "^2.0.0"
+      },
+      "peerDependencies": {
+        "react": ">=16.8.0"
+      }
+    },
     "node_modules/@emnapi/core": {
       "version": "1.8.1",
       "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.8.1.tgz",