feat(frontend): cap the Files grid at ~4 viewports with edge windowing

The grid grew without bound as you scrolled (and the section cache then
snapshotted the whole thing). It now keeps at most ~4 viewports of rows:
once it grows past the cap on one end, loadMore/loadPrev trim the
off-screen rows on the other end. The trimmed boundary cursor is dropped
and the opposite has-more flag is raised, so scrolling back refills that
side from an anchored window (?anchor=<file>), reusing the existing
prepend scroll-compensation. This bounds both live memory and the cached
snapshot regardless of how deep you scroll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/files/+page.svelte

@@ -339,14 +339,32 @@
 	// between) so there's no visible jump. Shares the `loading` guard with loadMore
 	// so the two never mutate files concurrently.
 	async function loadPrev() {
-		if (loading || !hasPrev || !prevCursor) return;
+		if (loading || !hasPrev) return;
 		loading = true;
 		try {
+			let items: File[];
+			let newPrevCursor: string | null;
+			if (prevCursor) {
 				const params = baseListParams();
 				params.set('cursor', prevCursor);
 				params.set('direction', 'backward');
 				const res = await api.get<FileCursorPage>(`/files?${params}`);
-			const items = res.items ?? [];
+				items = res.items ?? [];
+				newPrevCursor = res.prev_cursor ?? null;
+			} else {
+				// The head cursor was dropped when the window trimmed its top. Refetch
+				// the rows just before the current first file from an anchored window.
+				const firstId = files[0]?.id;
+				if (!firstId) {
+					hasPrev = false;
+					return;
+				}
+				const res = await fetchAnchorWindow(firstId);
+				const all = res.items ?? [];
+				const idx = all.findIndex((f) => f.id === firstId);
+				items = idx > 0 ? all.slice(0, idx) : [];
+				newPrevCursor = res.prev_cursor ?? null;
+			}
 			if (items.length === 0) {
 				hasPrev = false;
 				return;
@@ -359,11 +377,13 @@
 			const beforeHeight = scroller.scrollHeight;
 
 			files = [...items, ...files];
-			prevCursor = res.prev_cursor ?? null;
-			hasPrev = !!res.prev_cursor;
+			prevCursor = newPrevCursor;
+			hasPrev = !!newPrevCursor;
 
 			flushSync(); // apply the prepend now, before the browser paints
 			scroller.scrollTop = beforeTop + (scroller.scrollHeight - beforeHeight);
+
+			trimTail();
 		} catch {
 			hasPrev = false;
 		} finally {
@@ -385,17 +405,85 @@
 		return (document.scrollingElement as HTMLElement | null) ?? document.documentElement;
 	}
 
+	// ---- Windowing -----------------------------------------------------------
+	// The grid keeps at most ~4 viewports of rows in memory. As it grows past the
+	// cap on one end, the off-screen rows on the other end are trimmed; the cursor
+	// for the trimmed boundary is dropped (set null) and the opposite `has*` flag
+	// is raised, so scrolling back refills that side from an anchored window.
+
+	const CARD_PITCH = 162; // 160px thumbnail + 2px grid gap
+
+	function windowCap(): number {
+		const scroller = getScroller();
+		const w = scroller.clientWidth || 390;
+		const h = scroller.clientHeight || 700;
+		const cols = Math.max(1, Math.floor(w / CARD_PITCH));
+		const rows = Math.max(1, Math.ceil(h / CARD_PITCH));
+		return Math.max(4 * cols * rows, 2 * LIMIT);
+	}
+
+	// Fetch a window centred on a file (with its boundary cursors), used to refill
+	// a trimmed edge where the original cursor is no longer held.
+	function fetchAnchorWindow(anchorId: string): Promise<FileCursorPage> {
+		const a = baseListParams();
+		a.set('anchor', anchorId);
+		return api.get<FileCursorPage>(`/files?${a}`);
+	}
+
+	// Drop the off-screen rows above the viewport once the grid grew past the cap.
+	// Run after appended rows have painted so the height delta measures only the
+	// removed top; scroll is compensated so the visible rows don't jump.
+	function trimHead() {
+		const cap = windowCap();
+		if (files.length <= cap) return;
+		flushSync(); // paint the just-appended (below-fold) rows before measuring
+		const scroller = getScroller();
+		const beforeTop = scroller.scrollTop;
+		const beforeHeight = scroller.scrollHeight;
+		files = files.slice(files.length - cap);
+		prevCursor = null;
+		hasPrev = true;
+		flushSync();
+		scroller.scrollTop = beforeTop + (scroller.scrollHeight - beforeHeight);
+	}
+
+	// Symmetric to trimHead for upward growth: drop the off-screen rows below the
+	// viewport. No scroll compensation — the removed rows are past the fold.
+	function trimTail() {
+		const cap = windowCap();
+		if (files.length <= cap) return;
+		files = files.slice(0, cap);
+		nextCursor = null;
+		hasMore = true;
+	}
+
 	async function loadMore() {
 		if (loading || !hasMore) return;
 		loading = true;
 		error = '';
 		try {
+			let newItems: File[];
+			let newNextCursor: string | null;
+			if (nextCursor == null && files.length > 0) {
+				// The tail cursor was dropped when the window trimmed its bottom.
+				// Refetch the rows after the current last file from an anchored window.
+				const lastId = files[files.length - 1]?.id;
+				const res = await fetchAnchorWindow(lastId!);
+				const all = res.items ?? [];
+				const idx = all.findIndex((f) => f.id === lastId);
+				newItems = idx >= 0 ? all.slice(idx + 1) : [];
+				newNextCursor = res.next_cursor ?? null;
+			} else {
 				const params = baseListParams();
 				if (nextCursor) params.set('cursor', nextCursor);
 				const res = await api.get<FileCursorPage>(`/files?${params}`);
-			files = [...files, ...(res.items ?? [])];
-			nextCursor = res.next_cursor ?? null;
-			hasMore = !!res.next_cursor;
+				newItems = res.items ?? [];
+				newNextCursor = res.next_cursor ?? null;
+			}
+			files = [...files, ...newItems];
+			nextCursor = newNextCursor;
+			hasMore = !!newNextCursor;
+			trimHead();
 		} catch (err) {
 			error = err instanceof ApiError ? err.message : 'Failed to load files';
 			hasMore = false;