feat(ui): keyboard navigation and focus restore in album grid

- Cursor separate from selection; arrow keys move it, Space/Shift+Arrow/Enter/Esc act
- Click on empty grid space clears selection back to album properties
- Focus returns to the last viewed photo after closing the viewer

frontend/src/lib/components/PhotoGrid.svelte

@@ -6,16 +6,31 @@
 	interface Props {
 		photos: Photo[];
 		selectedIds?: string[];
+		cursorIndex?: number | null;
+		cursorVisible?: boolean;
 		onSelect?: (index: number, event: MouseEvent) => void;
 		onOpen?: (index: number) => void;
 		onDragStart?: (index: number, event: DragEvent) => void;
 		onContextMenu?: (index: number, event: MouseEvent) => void;
+		onCursorFocus?: (index: number) => void;
 	}
 
-	let { photos, selectedIds = [], onSelect, onOpen, onDragStart, onContextMenu }: Props =
-		$props();
+	let {
+		photos,
+		selectedIds = [],
+		cursorIndex = null,
+		cursorVisible = false,
+		onSelect,
+		onOpen,
+		onDragStart,
+		onContextMenu,
+		onCursorFocus
+	}: Props = $props();
 
 	const isSelected = (id: string) => selectedIds.includes(id);
+	const isCursor = (i: number) => cursorIndex === i;
+	const tabIndexFor = (i: number) =>
+		cursorIndex == null ? (i === 0 ? 0 : -1) : isCursor(i) ? 0 : -1;
 </script>
 
 {#if photos.length === 0}
@@ -29,13 +44,18 @@
 			<li>
 				<button
 					type="button"
+					data-photo-cell
+					tabindex={tabIndexFor(i)}
 					draggable={isSelected(photo.id)}
 					class="border-border bg-bg-muted group relative block aspect-square w-full overflow-hidden rounded-lg border {isSelected(
 						photo.id
 					)
 						? 'ring-primary ring-2'
+						: ''} {isCursor(i) && cursorVisible
+						? 'outline outline-2 outline-offset-2 outline-text'
 						: ''}"
 					aria-pressed={isSelected(photo.id)}
+					onfocus={() => onCursorFocus?.(i)}
 					onclick={(e) => onSelect?.(i, e)}
 					ondblclick={() => onOpen?.(i)}
 					ondragstart={(e) => onDragStart?.(i, e)}