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/routes/+layout.svelte

@@ -16,6 +16,7 @@
 	import IndexerStatusPill from '$lib/components/layout/IndexerStatusPill.svelte';
 	import LeftSidebar from '$lib/components/layout/LeftSidebar.svelte';
 	import AnimatedMule from '$lib/components/mule/AnimatedMule.svelte';
+	import PreviewModal from '$lib/components/preview/PreviewModal.svelte';
 
 	let { children } = $props();
 
@@ -108,6 +109,11 @@
 				</div>
 			</div>
 		</div>
+		<!-- Single shared preview modal. Reads view.previewOpen +
+		     selection.focused; any route can pop it via the openPreview
+		     helper (called by the timeline / PhotoGrid dblclick paths and
+		     by gridKeyNav's Space handler). -->
+		<PreviewModal />
 	{:else}
 		{@render children?.()}
 	{/if}