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/composables/useVirtualWindow.svelte.ts

@@ -14,10 +14,13 @@ export class VirtualWindow {
 	aboveBy = $state(0);
 	/** Height of the scrollable viewport in px. */
 	viewportH = $state(0);
+	/** Bumped on every resize so consumers can re-measure size-dependent layout. */
+	resizeTick = $state(0);
 
 	#root: HTMLElement | null = null;
 	#scroller: HTMLElement | null = null;
 	#ticking = false;
+	#resizing = false;
 
 	/** Nearest scrollable ancestor, or null to mean the window/document. */
 	#findScroller(el: HTMLElement): HTMLElement | null {
@@ -52,20 +55,30 @@ export class VirtualWindow {
 		});
 	};
 
+	#onResize = (): void => {
+		if (this.#resizing) return;
+		this.#resizing = true;
+		requestAnimationFrame(() => {
+			this.#resizing = false;
+			this.#measure();
+			this.resizeTick++;
+		});
+	};
+
 	/** Begin observing `root`; returns a teardown to call from `onMount`. */
 	observe(root: HTMLElement): () => void {
 		this.#root = root;
 		this.#scroller = this.#findScroller(root);
 		const target: EventTarget = this.#scroller ?? window;
 		target.addEventListener('scroll', this.#onScroll, { passive: true });
-		window.addEventListener('resize', this.#onScroll, { passive: true });
-		const ro = new ResizeObserver(this.#onScroll);
+		window.addEventListener('resize', this.#onResize, { passive: true });
+		const ro = new ResizeObserver(this.#onResize);
 		if (this.#scroller) ro.observe(this.#scroller);
 		ro.observe(root);
 		this.#measure();
 		return () => {
 			target.removeEventListener('scroll', this.#onScroll);
-			window.removeEventListener('resize', this.#onScroll);
+			window.removeEventListener('resize', this.#onResize);
 			ro.disconnect();
 		};
 	}