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/preview/VideoPlayer.svelte

@@ -106,10 +106,10 @@
 	:global(.vds-player) {
 		/* Let the player size to the video's intrinsic aspect ratio,
 		   capped by the host pane. Don't set width/height to 100% — that
-		   was stretching widescreen video into the square inline-preview
-		   pane. The default vidstack layout reads the loaded media's
-		   aspect and sizes the box accordingly; we just clamp the upper
-		   bound so it can't escape the SplitGrid top pane. */
+		   was stretching widescreen video into the square preview pane.
+		   The default vidstack layout reads the loaded media's aspect
+		   and sizes the box accordingly; we just clamp the upper bound
+		   so it can't escape its host pane. */
 		max-width: 100%;
 		max-height: 100%;
 		--media-brand: #3b82f6;