perf(frontend): virtualize ResourceList rows (list view)

Large folders/collections rendered every row into the DOM, so memory and
scripting time grew linearly with item count. A 20k-row list took ~12.5s to
first paint, held ~380k DOM nodes / ~281MB JS heap, and scrolled at ~4fps.

Add a reusable, dependency-free `VirtualList` that windows rows against the
nearest scrollable ancestor (the existing `.content-area`), so the
single-scrollbar UX, sticky header and end-of-list sentinel are unchanged. It
reserves the full scroll height with a sized spacer and translates a small
window of rows; row height is auto-measured for the single-column case.

Wire it into ResourceList's flat list view (recent, favorites, shared,
shared-with-me, trash, search). Grouped sections and grid view are unchanged
for now and are the next callers of the same primitive.

Measured in headless Chromium (1280x900) with synthetic rows, before/after:

  rows  | mount→paint | DOM nodes | JS heap | scroll frame | jank frames
  ------+-------------+-----------+---------+--------------+------------
   1000 | 632→70 ms   | 19k→351   | 16→2 MB | 17→17 ms     | 0→0
   5000 | 4401→54 ms  | 95k→351   | 72→3 MB | 47→17 ms     | 136→0
  20000 | 12577→56 ms | 380k→351  |281→6 MB |258→17 ms     | 1443→0

Rendered DOM and heap are now flat (O(visible)) regardless of dataset size;
scroll holds 60fps with zero jank. Correctness verified by probing a mid-list
scroll (rows land at the expected indices, positioned within the viewport).

eslint: disable core `no-undef` for `.svelte` (TypeScript/svelte-check already
resolve identifiers, including `<script generics>` type params it can't see).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M8Vb9QHmLZnEMzHz7MrFy6

frontend/src/lib/components/ResourceList.svelte

@@ -54,6 +54,7 @@
 	import EmptyState from '$lib/components/EmptyState.svelte';
 	import SkeletonList from '$lib/components/SkeletonList.svelte';
 	import ListToolbar from '$lib/components/ListToolbar.svelte';
+	import VirtualList from '$lib/components/VirtualList.svelte';
 	import { t } from '$lib/i18n/index.svelte';
 	import { files as filesStore } from '$lib/stores/files.svelte';
 	import { formatBytes } from '$lib/utils/format';
@@ -437,6 +438,10 @@
 						{@render row(entry)}
 					{/each}
 				{/each}
+			{:else if filesStore.viewMode === 'list'}
+				<!-- Flat list view: only the visible rows are mounted. The spacer keeps
+				     the full scroll height so the end-of-list sentinel still fires. -->
+				<VirtualList {items} rowHeight={56} key={(e) => e.id} {row} />
 			{:else}
 				{#each items as entry (entry.id)}
 					{@render row(entry)}