feat(ST-601): add loading skeletons for dashboard, settings and todos

De drie zware split-pane routes hadden al loading.tsx; dashboard,
settings en todos kregen elk een passende animate-pulse skeleton.

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

app/(app)/settings/loading.tsx

@@ -0,0 +1,10 @@
+export default function Loading() {
+  return (
+    <div className="p-6 max-w-2xl mx-auto w-full space-y-4 animate-pulse">
+      <div className="h-6 w-28 bg-border rounded" />
+      {[1, 2, 3, 4].map(i => (
+        <div key={i} className="h-28 bg-border/50 rounded-xl" />
+      ))}
+    </div>
+  )
+}