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/components/layout/LeftSidebar.tsx

@@ -11,6 +11,7 @@ import {
   Copy,
   Tag as TagIcon,
   Palette,
+  MapPin,
   Layers2,
   MoreHorizontal,
   Pencil,
@@ -248,6 +249,9 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
       case 'colors':
         navigateToSection('colors', { groupBy: 'color' })
         break
+      case 'map':
+        navigateToSection('map', {})
+        break
       default:
         if (id.startsWith('folder-')) {
           const folderId = id.slice('folder-'.length)
@@ -349,6 +353,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: stats?.rated ?? 0 },
         { id: 'tags', label: 'Tags', icon: <TagIcon className="h-4 w-4" />, count: tagsTotalCount },
         { id: 'colors', label: 'Colors', icon: <Palette className="h-4 w-4" />, count: stats?.colored ?? 0 },
+        { id: 'map', label: 'Map', icon: <MapPin className="h-4 w-4" />, count: stats?.with_gps ?? 0 },
         { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: stats?.duplicates ?? 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: stats?.discarded ?? 0 },
       ],