fix(documents): cleanup timeline drag listeners on unmount (#385)

Pointerdown attaches three document-level listeners. Without an
explicit teardown, an unmount mid-drag (route change, view toggle,
viewport drops below lg) left them attached and they kept writing
to torn-down state cells.

Wrap the cleanup in $effect's return, which Svelte 5 invokes on
unmount. The listener-removal regression test pins this so the bug
cannot come back silently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/TimelineDensityFilter.svelte

@@ -160,6 +160,13 @@ function cleanupDragListeners() {
 	document.removeEventListener('pointercancel', handleDocumentCancel);
 }
 
+// Strip any in-flight document listeners if the component unmounts mid-drag
+// (route change, view toggle, breakpoint drop). Without this they survive on
+// document and keep writing to torn-down state cells.
+$effect(() => {
+	return cleanupDragListeners;
+});
+
 function finalizeDrag() {
 	if (dragStartIndex === null || dragEndIndex === null) return;
 	const start = dragStartIndex;

frontend/src/lib/document/TimelineDensityFilter.svelte.spec.ts

@@ -389,6 +389,30 @@ describe('TimelineDensityFilter — aria-live during drag', () => {
 	});
 });
 
+describe('TimelineDensityFilter — listener cleanup on unmount', () => {
+	it('removes document pointer listeners when unmounted mid-drag', async () => {
+		const removed: string[] = [];
+		const realRemove = document.removeEventListener.bind(document);
+		const removeSpy = vi
+			.spyOn(document, 'removeEventListener')
+			.mockImplementation((type: string, listener, options) => {
+				removed.push(type);
+				return realRemove(type, listener as EventListener, options);
+			});
+
+		render(TimelineDensityFilter, makeProps());
+		const bar = document.querySelector('[data-testid="timeline-bar"]') as HTMLElement;
+		bar.dispatchEvent(new PointerEvent('pointerdown', { bubbles: true, pointerId: 1, button: 0 }));
+
+		cleanup();
+
+		expect(removed).toContain('pointermove');
+		expect(removed).toContain('pointerup');
+		expect(removed).toContain('pointercancel');
+		removeSpy.mockRestore();
+	});
+});
+
 describe('TimelineDensityFilter — drag-to-select-range', () => {
 	function pointerDown(el: HTMLElement) {
 		const event = new PointerEvent('pointerdown', { bubbles: true, pointerId: 1, button: 0 });