layoutStore.ts: 1,485 lines mixing Yjs CRDT sync with layout state

## Recommendation

### ♻️ REFACTOR: Extract Yjs sync into a dedicated composable

The layout store mixes CRDT synchronization logic with layout state management.

## What Was Found

- **File:** `src/renderer/core/layout/store/layoutStore.ts` — 1,485 lines
- **Consumers:** 21 files
- **Mixed concerns:** Yjs CRDT sync logic + layout state management

## Context

Extract Yjs sync into a dedicated `useYjsSync` composable. Keep the store thin and focused on layout state. The Yjs sync logic is independently testable.

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**Category:** Refactoring Opportunity | **Confidence:** MEDIUM | **Tools:** Static analysis
_Filed by audit-code skill_

Part of #11022

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11038-layoutStore-ts-1-485-lines-mixing-Yjs-CRDT-sync-with-layout-state-33e6d73d365081638feef3dc6a9cc414) by [Unito](https://www.unito.io)