fix: load tasks before first render, not in onMount

Removing the splash exposed something it had been covering. Tasks loaded
in onMount, so every launch rendered one frame of the empty state first.

localStorage is synchronous, so there was never a reason to wait.
Migration and rollover run during initialisation and the first render has
the real list. onMount keeps only the rollover triggers.

src/App.svelte

@@ -13,14 +13,21 @@
   const storage = createStorage();
   const undoStack = createUndoStack();
 
-  let tasks = $state([]);
+  // Storage is synchronous, so the first render can already have the real list.
+  // Loading in onMount instead meant one frame of the empty state on every
+  // launch — previously hidden behind the splash screen, and very visible once
+  // that went away.
+  storage.migrateLegacyKeys();
+  const bootNow = new Date();
+
+  let tasks = $state(rollover(storage, bootNow));
   let newTask = $state('');
   // Seeded from the class the pre-paint script in index.html already set, so
   // there is one source of truth and no post-mount correction to flash.
   let darkMode = $state(document.documentElement.classList.contains('dark'));
   let isInitialized = false;
-  let todayKey = $state(toKey(new Date()));
-  let selectedKey = $state(toKey(new Date()));
+  let todayKey = $state(toKey(bootNow));
+  let selectedKey = $state(toKey(bootNow));
   let draggedItem = $state(null);
   let draggedOverIndex = $state(null);
   let midnightTimer = null;
@@ -286,8 +293,8 @@
   });
 
   onMount(() => {
-    storage.migrateLegacyKeys();
-    runRollover();
+    // State is already loaded above; this only opens the write path and lets
+    // the theme effect run once without persisting on a plain page load.
     isInitialized = true;
 
     // Four triggers, because no single one is sufficient. The timer covers a