feat(electric): add offline persistence adapter for collections

.changeset/add-offline-persistence.md

@@ -0,0 +1,29 @@
+---
+"@tanstack/electric-db-collection": minor
+---
+
+feat: Add offline persistence adapter for Electric collections
+
+This adds a localStorage-based persistence layer that enables offline-first data access and faster initial loads.
+
+**What changed:**
+- New `persistence` configuration option in `electricCollectionOptions`
+- Collection data is automatically saved to localStorage as it syncs
+- Persisted data is restored on collection initialization
+- Shape handle and offset are persisted for resumable sync
+- In `on-demand` mode, collections are marked ready immediately after loading from persistence
+
+**Why:**
+Enables offline-first applications where users can see their data immediately on app launch, even before network sync completes. This improves perceived performance and allows the app to function without network connectivity.
+
+**How to use:**
+```typescript
+electricCollectionOptions({
+  id: 'my-collection',
+  syncMode: 'on-demand',
+  persistence: {
+    storageKey: 'my-collection-storage',
+  },
+  // ... other options
+})
+```