fix: Treat a stored JSON null as a missing persisted value

readJSON returned any successfully parsed value, so a localStorage
entry holding the literal "null" came back as null instead of the
fallback — and a null inputState crashes module init on every load
until storage is cleared. The deleted MacFJA persist layer guarded
this with `null !== initialValue`; restore that behavior.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib/util/persist.svelte.test.ts

@@ -22,6 +22,12 @@ describe('readJSON', () => {
     window.localStorage.setItem('legacy', 'undefined');
     expect(readJSON('legacy', 'fallback')).toBe('fallback');
   });
+
+  it('returns the fallback when the stored value parses to null', () => {
+    // The pre-runes persistence layer treated a stored null as absent.
+    window.localStorage.setItem('legacy-null', 'null');
+    expect(readJSON('legacy-null', 'fallback')).toBe('fallback');
+  });
 });
 
 describe('writeJSON', () => {
@@ -51,4 +57,10 @@ describe('persisted', () => {
     expect(counter.value).toBe(42);
     expect(window.localStorage.getItem('counter')).toBe('42');
   });
+
+  it('uses the initial value when storage holds a literal null', () => {
+    window.localStorage.setItem('settings', 'null');
+    const settings = persisted('settings', { theme: 'default' });
+    expect(settings.value).toEqual({ theme: 'default' });
+  });
 });

src/lib/util/persist.svelte.ts

@@ -15,7 +15,12 @@ export const readJSON = <T>(key: string, fallback: T): T => {
   }
   try {
     const raw = window.localStorage.getItem(key);
-    return raw === null ? fallback : (JSON.parse(raw) as T);
+    if (raw === null) {
+      return fallback;
+    }
+    // A stored literal "null" means the value is absent: the pre-runes
+    // persistence layer never wrote null and treated it as missing.
+    return (JSON.parse(raw) as T) ?? fallback;
   } catch {
     return fallback;
   }