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/layout/FolderTree.svelte

@@ -120,11 +120,11 @@
 				with the px-2 of Views/Heaps rows; +12px per nested level.
 			-->
 			<div
-				class="group flex h-[24px] items-center rounded pr-2 text-[12px] leading-tight hover:bg-accent"
+				class="group flex h-[22px] items-center rounded pr-2 text-[12px] leading-tight hover:bg-accent"
 				class:bg-primary={active}
 				class:text-primary-foreground={active}
 				class:hover:bg-primary={active}
-				style="padding-left: {8 + depth * 12}px;"
+				style="padding-left: {4 + depth * 12}px;"
 			>
 				{#if hasChildren}
 					<button
@@ -136,10 +136,10 @@
 					>
 						{open ? '▾' : '▸'}
 					</button>
-				{:else if depth > 0}
+				{:else}
 					<!-- Spacer keeps childless siblings aligned with their chevroned
-					     peers at nested depths. Skipped at depth 0 so root folders
-					     left-align with the Views/Heaps rows. -->
+					     peers at every depth, so labels share a common left edge
+					     across the sidebar (folders, heaps, views, manage). -->
 					<span class="inline-block h-[18px] w-4" aria-hidden="true"></span>
 				{/if}
 				<!--
@@ -148,8 +148,7 @@
 					zone right where the user's eye lands.
 				-->
 				<button
-					class="flex min-w-0 flex-1 items-center text-left"
-					class:px-1={hasChildren || depth > 0}
+					class="flex min-w-0 flex-1 items-center pl-1 text-left"
 					onclick={() => onPick(node.path)}
 					ondblclick={readonly ? undefined : () => onRename?.(node.path)}
 					title={node.path}
@@ -158,7 +157,7 @@
 					{#if counts && counts[node.path] !== undefined}
 						{@const n = counts[node.path]}
 						<span
-							class="ml-auto shrink-0 rounded px-1 text-[10px] tabular-nums {active
+							class="ml-auto flex h-4 min-w-[24px] flex-shrink-0 items-center justify-center rounded px-1 text-[10px] tabular-nums {active
 								? 'bg-primary-foreground/15 text-primary-foreground'
 								: 'bg-secondary text-muted-foreground'}"
 						>