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

@@ -33,3 +33,6 @@ footer:
   instagram: quasiralf
 #  email: hello@example.com
 #  website: https://example.com
+
+# Enable the /map page
+map: true