feat(electric): add offline persistence adapter for collections

[TomasGonzalez]

🎯 Changes

Add offline persistence adapter for Electric collections
This PR introduces a localStorage-based persistence layer for Electric collections, enabling offline-first data access and faster initial loads.

Key features:

• Persist synced data locally: Collection data is automatically saved to localStorage as it syncs, configurable via persistence.storageKey

• Restore on startup: Persisted data is loaded into the collection on initialization, making data available immediately without waiting for network

• Resumable sync: Shape handle and last offset are persisted, allowing sync to resume from where it left off

Use case: This enables offline-first apps where users can see their data immediately on app launch, even before the network sync completes.

Configuration:

  electricCollectionOptions({
    id: 'my-collection',
    syncMode: 'on-demand',
    persistence: {
      storageKey: 'my-collection-storage',
    },
    // ... other options
  })

✅ Checklist

• (not found) I have followed the steps in the Contributing guide. (not found)
• I have tested this code locally with pnpm test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

🦋 Changeset detected

Latest commit: e550691

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@tanstack/electric-db-collection	Minor
@tanstack/db-collection-e2e	Patch
@tanstack/db-example-solid-todo	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR