feat(frontend): port Places, People & photo tabs to the Svelte app

Bring the Photos/People/Places UI that main added (in the legacy vanilla
frontend) into the SvelteKit rewrite, wired to the now-merged backend
(/api/photos/geo, /api/people/*).

Photos page (routes/photos/+page.svelte):
- Moments | Places | People sub-tabs (the People tab appears only when the
  faces feature is enabled, via a /api/people capability probe), mirroring
  the vanilla photos sub-nav.
- Square ↔ justified layout toggle. Justified uses a Flickr-style
  row-packer over the width/height the photos list endpoint returns
  (PhotoItem), falling back to 1:1 when dimensions are missing.

New components:
- PhotoLightbox.svelte — the lightbox extracted from the photos page into a
  reusable component (items + bindable index, onDelete callback) so the
  grid, People and Places all share one implementation (no duplication).
- PlacesMap.svelte — MapLibre GL map with server-clustered markers; the
  vector basemap is optional (probed at /basemaps/basemap.pmtiles, themed
  fallback otherwise). Cluster click zooms in or opens the lightbox.
- PeopleView.svelte — identity-cluster grid → per-person photo grid, with
  rename via the in-app prompt dialog.

Supporting:
- api/endpoints/people.ts (+ peopleEnabled probe); photos.ts gains
  fetchPhotosGeo + GeoCluster + PhotoItem; fileThumbnailUrl takes a size.
- lib/vendor/maplibre.ts — minimal typings + lazy loader for the vendored
  MapLibre GL + pmtiles globals (kept any-free for ESLint).
- utils/media.ts — shared isVideo / photoTimestamp / minimalPhotoItem.
- Vendored maplibre-gl 5.24.0 + pmtiles 4.4.1 under static/vendors and an
  optional static/basemaps dir, matching the PR's vendored-asset pattern.
- New photos.tab_*/layout_*/map_* + people.* keys in en.json.

Verified: npm run check (svelte-check + eslint + stylelint + prettier),
npm run test:unit (36 pass), and npm run build all green.

frontend/src/lib/utils/media.ts

@@ -0,0 +1,42 @@
+/** Shared helpers for the photo/video timeline (used by the grid, lightbox,
+ * People and Places views). */
+import type { FileItem } from '$lib/api/types';
+
+/** True for video tiles (they get a play badge and client-side frame thumbs). */
+export function isVideo(p: FileItem): boolean {
+	return (p.mime_type ?? '').startsWith('video/');
+}
+
+/**
+ * EXIF-aware capture timestamp in milliseconds. `sort_date`/`created_at` are
+ * stored in seconds; values below ~1e12 are treated as seconds and scaled up.
+ */
+export function photoTimestamp(p: FileItem): number {
+	const v = p.sort_date || p.created_at || 0;
+	return v < 1e12 ? v * 1000 : v;
+}
+
+/**
+ * Build a minimal {@link FileItem} from just an id — used by People and Places
+ * to open the lightbox by id and let it lazily fetch the rest (name, EXIF).
+ */
+export function minimalPhotoItem(id: string): FileItem {
+	return {
+		category: 'image',
+		created_at: 0,
+		icon_class: '',
+		icon_special_class: '',
+		id,
+		mime_type: 'image/jpeg',
+		modified_at: 0,
+		name: '',
+		owner_id: '',
+		folder_id: '',
+		path: '',
+		size: 0,
+		size_formatted: '',
+		sort_date: 0,
+		etag: '',
+		content_hash: ''
+	};
+}