feat(planner): add AbortController to suggestion fetch $effect

Cancels the inflight request when activePickerDate changes or picker
closes, preventing stale responses from overwriting suggestions.
Adds page.test.ts covering fetch trigger, suggestion rendering,
and AbortSignal presence.

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

frontend/src/routes/(app)/planner/+page.svelte

@@ -106,12 +106,14 @@
 			isLoadingSuggestions = false;
 			return;
 		}
+		const controller = new AbortController();
 		isLoadingSuggestions = true;
-		fetch(`/planner?planId=${weekPlan.id}&date=${activePickerDate}`)
+		fetch(`/planner?planId=${weekPlan.id}&date=${activePickerDate}`, { signal: controller.signal })
 			.then((r) => r.json())
 			.then((d) => { suggestions = d.suggestions ?? []; })
-			.catch(() => { suggestions = []; })
+			.catch((e) => { if (e.name !== 'AbortError') suggestions = []; })
 			.finally(() => { isLoadingSuggestions = false; });
+		return () => controller.abort();
 	});
 
 	function handleSelectDay(day: string) {