feat: migrate some more stuff to svelte-5, mainly app settings

app/src/lib/helpers/localState.svelte.ts

@@ -0,0 +1,11 @@
+export function localState<T>(key: string, defaultValue: T): T {
+  const stored = localStorage.getItem(key)
+  const state = $state(stored ? JSON.parse(stored) : defaultValue)
+  $effect.root(() => {
+    $effect(() => {
+      const value = $state.snapshot(state);
+      localStorage.setItem(key, JSON.stringify(value));
+    });
+  });
+  return state;
+}