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/BulkActionBar.svelte

@@ -13,6 +13,7 @@
 	} from '$lib/services/photoprism';
 	import { batchEdit } from '$lib/services/batch';
 	import {
+		clearBulkToFirst,
 		clearSelection,
 		focusAfter,
 		selection,
@@ -59,6 +60,10 @@
 	const isArchive = $derived(filters.section === 'archive');
 
 	function clearAll() {
+		// Mirror gridKeyNav's Esc: a multi-selection collapses back to its
+		// first member (the user keeps a single-focus reference) before
+		// the next Clear/Esc fully dismisses focus.
+		if (clearBulkToFirst()) return;
 		clearSelection();
 		setFocused(null);
 	}