feat(frontend): open file viewer as overlay over the mounted list

The viewer was a separate /files/[id] route, so returning tore down and
reloaded the whole grid. Now opening a file uses SvelteKit shallow
routing (pushState + page.state.fileId): the list stays mounted and the
viewer renders as a full-screen overlay on top of it, like Immich. The
URL still becomes /files/<id> and the back button (or Escape/close)
dismisses the overlay via history.back(), revealing the untouched grid —
no reload — then scrolls it to the last-viewed file instantly.

- Extract the viewer UI/logic into a reusable FileViewer component
  (file fetch, preview, lazy tags, save, prev/next, keyboard).
- List: neighbours come straight from its own files[]; paging past the
  loaded set pulls the next page by cursor (prefetch near the end).
- Paging uses replaceState so one back press returns to the grid.
- /files/[id] remains as a thin standalone fallback for deep links /
  hard reloads, resolving neighbours via the anchor API and returning
  to the grid with ?anchor=<id>.
- Remove the now-unused filesCache snapshot store (the list is never
  unmounted, so there's nothing to snapshot/restore).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app.d.ts

@@ -5,7 +5,11 @@ declare global {
 		// interface Error {}
 		// interface Locals {}
 		// interface PageData {}
-		// interface PageState {}
+		interface PageState {
+			/** Set via shallow routing when the file viewer is open as an overlay
+			 *  on top of the files list. */
+			fileId?: string;
+		}
 		// interface Platform {}
 	}
 }