fix(shared): give createPersistentStore a destroy() to dispose its effect.root

The store created an $effect.root for the save-on-change sync but returned no
disposer, so the effect leaked for the life of the process — contradicting
the rule that $effect.root owners must expose destroy(). Capture and expose
the disposer.

- add destroy() to the returned store; covered by tests (flushSync proves the
  save effect runs before destroy and stops after)
- trim the bloated header (two near-duplicate @example blocks) to one concise
  JSDoc — no fluff
- update typographySettings test mocks to satisfy the now-required destroy()

Consumers (LayoutManager, ThemeManager, typographySettings, comparisonStore)
do not yet call it — threading + the createSingleton migration follow.

src/shared/lib/helpers/createPersistentStore/createPersistentStore.test.ts

@@ -1,6 +1,7 @@
 /**
  * @vitest-environment jsdom
  */
+import { flushSync } from 'svelte';
 import {
     afterEach,
     beforeEach,
@@ -376,4 +377,39 @@ describe('createPersistentStore', () => {
             expect(store.value[0].name).toBe('First');
         });
     });
+
+    describe('Lifecycle', () => {
+        it('persists value changes via the sync effect', () => {
+            const store = createPersistentStore(testKey, 'a');
+            const spy = vi.spyOn(mockLocalStorage, 'setItem');
+
+            store.value = 'b';
+            flushSync();
+
+            expect(spy).toHaveBeenCalledWith(testKey, JSON.stringify('b'));
+        });
+
+        it('stops persisting after destroy()', () => {
+            const store = createPersistentStore(testKey, 'a');
+            flushSync();
+            store.destroy();
+
+            const spy = vi.spyOn(mockLocalStorage, 'setItem');
+            store.value = 'c';
+            flushSync();
+
+            expect(spy).not.toHaveBeenCalled();
+            // reading still works after disposal
+            expect(store.value).toBe('c');
+        });
+
+        it('destroy() is safe to call repeatedly', () => {
+            const store = createPersistentStore(testKey, 'a');
+
+            expect(() => {
+                store.destroy();
+                store.destroy();
+            }).not.toThrow();
+        });
+    });
 });