preview: full-screen modal replaces inline split + tags route reorg

The old SplitGrid + InlinePreview pane is replaced by a full-screen
PreviewModal mounted once at the layout root. Open via Space on the
focused tile or double-click; close on Esc (or X / Space again).
Inside, PreviewPane renders the focused photo, RightSidebar carries
the metadata, BulkActionBar reuses the existing per-photo actions,
and PreviewCarousel windows ±50 thumbs around the focused index.

Selection contract matches the grid: plain click reduces, shift
extends the range, ⌘/Ctrl toggles, plain arrow drops the multi-
selection, shift-arrow extends. New clearBulkToFirst() helper makes
Esc / Clear collapse a bulk back to single-focus on its first member
before the next press fully dismisses (modal closes, grid clears
focus).

Tags route reorganised into /tags/[category]/[[value]] with its own
+layout and TagsBrowserSidebar; the old monolithic /tags/+page is
trimmed to a legacy redirect.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/timeline/PhotoGrid.svelte

@@ -21,7 +21,7 @@
 		setFocused,
 		setOrder
 	} from '$lib/stores/selection.svelte';
-	import { view } from '$lib/stores/view.svelte';
+	import { openPreview, view } from '$lib/stores/view.svelte';
 	import { type PpPhoto } from '$lib/types/photoprism';
 	import PhotoTile from './PhotoTile.svelte';
 
@@ -80,6 +80,7 @@
 		selection.ids.add(uid);
 		setFocused(uid);
 		setAnchor(uid);
+		openPreview();
 	}
 </script>