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/App.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react'
 import { Timeline } from './components/timeline/Timeline'
 import { DuplicatesView } from './components/duplicates/DuplicatesView'
+import { MapView } from './components/map/MapView'
 import { LeftSidebar } from './components/layout/LeftSidebar'
 import { RightSidebar } from './components/layout/RightSidebar'
 import { TopBar } from './components/layout/TopBar'
@@ -73,11 +74,17 @@ function App() {
           <FilterBar />
           <DiscardActionBar />
           <div className="flex-1 overflow-auto">
-            {/* The Duplicates section gets its own grouped grid view —
-             *  the regular timeline can't represent groups, and a flat
-             *  filtered list of "is_duplicate=true" photos was the old
-             *  half-broken UX. */}
-            {currentSection === 'duplicates' ? <DuplicatesView /> : <Timeline />}
+            {/* Section-level routing. The Map view replaces the timeline
+             *  with a Leaflet map of GPS-tagged photos; Duplicates gets its
+             *  own grouped grid; everything else falls through to the
+             *  filter-driven Timeline. */}
+            {currentSection === 'map' ? (
+              <MapView />
+            ) : currentSection === 'duplicates' ? (
+              <DuplicatesView />
+            ) : (
+              <Timeline />
+            )}
           </div>
           {/* Floating keyboard hints — bottom-center of the main column,
            *  glassy. Mounted here so it's centered against the timeline,