[WIP] feat(connector): automerge connector demo

[ulfgebhardt]

All done. Here's a summary of what was implemented:

Part 1: createdAt: Date → string

• data-interface/src/index.ts — createdAt: string in Item interface
• data-interface/src/demo-data.ts — simplified to direct cast (no new Date() conversion)
• mock-connector/src/mock-connector.ts — new Date().toISOString()
• graphql-server/src/store.ts — removed Date replacer from clone(), removed .map() Date conversion, uses .toISOString()
• graphql-server/src/schema/scalars.ts — DateTime scalar now works with strings only
• graphql-connector/src/graphql-connector.ts — parseItem passes through as-is

Part 2: stripNulls deduplicated

• Created graphql-server/src/schema/utils.ts with shared stripNulls
• Updated imports in queries.ts and subscriptions.ts

Part 3: Automerge-Connector

• packages/automerge-connector/ with full DataInterface implementation
• Uses @automerge/automerge-repo + IndexedDBStorageAdapter for persistence
• Root-Doc / Group-Doc architecture (one doc per group)
• Demo data seeded on first run, URL persisted in localStorage
• Reactive observe()/observeItem() via handle.on("change")
• All 8 packages build successfully with zero type errors

Based on #2