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

content/gallery.yaml.example

@@ -20,6 +20,9 @@ albums:
 # Show camera/lens EXIF on photo hover (default: true)
 # exifOnHover: false
 
+# Enable the /map page — plots photos on a world map by city/country (default: false)
+# map: true
+
 # Photo grid layout — customize columns, gap, aspect ratio, and mode.
 # grid:
 #   columns: 3        # default: 3