feat: map view with GPS extraction fix

Adds a new Map sidebar entry that plots photos by their EXIF GPS
coordinates on a clustered Leaflet map. While wiring this up, the
metadata extractor was reading unprefixed GPS keys that never exist
in `exiftool -G -j` output AND assumed coordinates were already
floats — every photo silently lost its GPS. The new extract_gps
helper handles Composite/EXIF group prefixes and parses DMS strings,
and lat/lon are stored as first-class indexed columns so the map
can query them without parsing exif_json on every request.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/main.tsx

@@ -4,6 +4,10 @@ import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import { ReactQueryDevtools } from '@tanstack/react-query-devtools'
 import App from './App'
 import './index.css'
+// Leaflet ships its base styles separately; without these the map tiles
+// render at 0×0 and marker icons go missing. Imported once globally so
+// any lazy MapView render picks them up.
+import 'leaflet/dist/leaflet.css'
 
 const queryClient = new QueryClient({
   defaultOptions: {