fix(todo): homepage loading state + completed date on task items

Homepage fixes:
- Add loading state: show TaskListSkeleton while liveQuery loads
- Fix $derived(() => ...) anti-pattern → $derived.by()
- Stabilize date calculations (compute once, not per re-render)
- Remove double error check (mutation errors shown via toast)

TaskItem improvements:
- Show completed-at date (small, 50% opacity) on right side of
  completed tasks
- Click completed date to toggle showing created-at date above it

Shared: Add `loading` field to useLiveQueryWithDefault (was missing,
prevented proper loading states in consumers)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/local-store/src/svelte/reactive.svelte.ts

@@ -82,8 +82,9 @@ export function useLiveQuery<T>(querier: () => T | Promise<T>): LiveQueryResult<
 export function useLiveQueryWithDefault<T>(
 	querier: () => T | Promise<T>,
 	defaultValue: T
-): { readonly value: T; readonly error: unknown } {
+): { readonly value: T; readonly loading: boolean; readonly error: unknown } {
 	let value = $state<T>(defaultValue);
+	let loading = $state(true);
 	let error = $state<unknown>(undefined);
 
 	const observable: Observable<T> = liveQuery(querier);
@@ -91,10 +92,12 @@ export function useLiveQueryWithDefault<T>(
 	const subscription = observable.subscribe({
 		next: (result) => {
 			value = result;
+			loading = false;
 			error = undefined;
 		},
 		error: (err) => {
 			error = err;
+			loading = false;
 		},
 	});
 
@@ -106,6 +109,9 @@ export function useLiveQueryWithDefault<T>(
 		get value() {
 			return value;
 		},
+		get loading() {
+			return loading;
+		},
 		get error() {
 			return error;
 		},