polish(document-detail): address review concerns on onMount deep-link wiring

Three small refinements from Felix's review cycle 1:

- replaceState(page.url.pathname, page.state ?? {}) — defend against
  first-navigation cases where page.state can be undefined.
- Extract the inline tick + requestAnimationFrame into a named
  waitForPanelRender() helper; intent is now readable from onMount.
- Attach .catch() to the fire-and-forget scrollToCommentFromQuery
  promise so any helper throw surfaces via console.error instead
  of silently disappearing.

No behavior change on the happy path. All existing tests stay green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -291,6 +291,11 @@ $effect(() => {
 
 let navHeight = $state(0);
 
+async function waitForPanelRender(): Promise<void> {
+	await tick();
+	await new Promise<void>((resolve) => requestAnimationFrame(() => resolve()));
+}
+
 onMount(() => {
 	navHeight = document.querySelector('header')?.getBoundingClientRect().height ?? 0;
 
@@ -311,13 +316,10 @@ onMount(() => {
 			setTimeout(() => (flashAnnotationId = null), prefersReducedMotion ? 2000 : 1500);
 		},
 		prefersReducedMotion,
-		afterTick: async () => {
-			await tick();
-			await new Promise<void>((resolve) => requestAnimationFrame(() => resolve()));
-		},
+		afterTick: waitForPanelRender,
 		getElement: (id) => document.getElementById(id),
-		onStripUrl: () => replaceState(page.url.pathname, page.state)
-	});
+		onStripUrl: () => replaceState(page.url.pathname, page.state ?? {})
+	}).catch((e) => console.error('deep-link scroll failed', e));
 
 	function onKeyDown(e: KeyboardEvent) {
 		if (e.key === 'Escape' && transcribeMode) {