feat: add smart map view with location-level clustering

- /map page with Leaflet (CartoDB dark tiles)
- Photos clustered by city/country (not per-photo)
- /api/map endpoint with encrypted cover asset IDs
- Configurable via gallery.yaml (map: true/false)
- Conditional nav link in header
- CSP updated for Leaflet CDN and tile servers
- Theme-aware popups with cover thumbnails and album links

package.json

@@ -15,8 +15,10 @@
     "test:e2e": "playwright test"
   },
   "dependencies": {
+    "@types/leaflet": "^1.9.21",
     "gray-matter": "^4.0.3",
     "js-yaml": "^4.1.1",
+    "leaflet": "^1.9.4",
     "next": "16.1.6",
     "react": "19.2.3",
     "react-dom": "19.2.3",