perf(frontend): virtualize grid views + the files browser (list & grid)

Extends windowing to the remaining O(n)-DOM surfaces: the card-grid view of
ResourceList (recent / favorites / shared / shared-with-me / trash / search) and
the main file browser (`files/[...path]`), in both list and grid layouts.

- VirtualList gains a real grid mode: its inner window carries the caller's grid
  class (`files-grid-view`) and lays out `columns` cards per windowed row. The
  row pitch is auto-measured (grid card height tracks column width via the 4/3
  aspect-ratio thumbnail) and re-measured on resize.
- `useVirtualWindow` now distinguishes scroll from resize and exposes a
  `resizeTick`, so size-dependent layout (the grid pitch) is recomputed only when
  it can actually change.
- `gridColumns(width)` (new util) mirrors the CSS `auto-fill` / `--grid-card-min`
  / gap so the windowed row count matches the browser's real wrapping exactly;
  shared by both grid callers.
- The files browser flattens folders-then-files into one discriminated `entries`
  list rendered through VirtualList (list: columns=1; grid: columns from width).
  Grouped (swimlane) views stay fully rendered, as before — they're bounded.

ResourceList GRID, headless Chromium (1280x900), synthetic rows, before/after:

  rows  | mount→paint | DOM nodes | JS heap | scroll frame | jank frames
  ------+-------------+-----------+---------+--------------+------------
   1000 | 979→75 ms   | 19k→879   | 16→3 MB | 16→17 ms     | 0→0
   5000 | 4005→67 ms  | 95k→879   | 72→3 MB | 37→17 ms     | 26→0
  20000 |12015→65 ms  | 380k→879  |281→7 MB |197→17 ms     |1249→19

Rendered DOM and heap are flat (O(visible)) regardless of dataset size; mount is
~185x faster and scroll holds ~60fps. Verified visually mid-scroll (5 columns,
4/3 thumbnail tiles, cards land at the expected indices). The files browser
shares the same VirtualList path (it can't be mounted headless — it depends on
$app routing/session — so it's validated via svelte-check + the production build
+ the shared, separately-benchmarked component).

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

frontend/src/lib/components/VirtualList.svelte

@@ -64,12 +64,17 @@
 	const offsetY = $derived(firstRow * effRowH);
 	const visible = $derived(items.slice(startIndex, endIndex));
 
-	/** Single-column: adopt the real rendered row height once it's known. */
+	/**
+	 * Adopt the real rendered row pitch once rows exist. For a grid (cols > 1) the
+	 * card height tracks the column width (e.g. an aspect-ratio thumbnail), so the
+	 * pitch is the card height plus the grid's row gap, re-measured on resize.
+	 */
 	function refineRowHeight(): void {
-		if (cols !== 1 || !rootEl) return;
-		const firstChild = rootEl.querySelector('.vlist__window > *') as HTMLElement | null;
-		if (!firstChild) return;
-		const h = firstChild.getBoundingClientRect().height;
+		const win = rootEl?.querySelector('.vlist__window') as HTMLElement | null;
+		const firstChild = win?.firstElementChild as HTMLElement | null;
+		if (!win || !firstChild) return;
+		let h = firstChild.getBoundingClientRect().height;
+		if (cols > 1) h += parseFloat(getComputedStyle(win).rowGap) || 0;
 		if (h > 0 && Math.abs(h - measuredRow) > 0.5) measuredRow = h;
 	}
 
@@ -82,9 +87,12 @@
 		return stop;
 	});
 
-	// Refine the measured row height once rows are actually in the DOM.
+	// Re-measure the row pitch when rows first render, columns change, or a resize
+	// reflows the cards (grid card height depends on the column width).
 	$effect(() => {
 		void visible.length;
+		void cols;
+		void vw.resizeTick;
 		refineRowHeight();
 	});
 </script>