refactor: move doc-title and ai-chat-block components (#10316)

### TL;DR
Moved doc title and AI chat block components to more appropriate locations while removing unused backlink functionality.

### What changed?
- Relocated doc title component from presets to affine-components
- Moved AI chat block from presets/blocks to blocks directory
- Removed unused backlink-related code and components
- Updated imports across files to reference new component locations
- Consolidated AI-related exports through a single entry point

### How to test?
1. Verify doc title still renders correctly in documents
2. Confirm AI chat functionality works as expected
3. Check that no backlink-related features are accessible
4. Ensure all AI features continue to work through the new import paths

### Why make this change?
This reorganization improves code organization by:
- Placing components closer to their related functionality
- Removing dead/unused code around backlinks
- Simplifying the import structure for AI-related features
- Making the codebase more maintainable by consolidating related components

packages/frontend/core/src/modules/cloud/services/auth.ts

@@ -1,4 +1,4 @@
-import { AIProvider } from '@affine/core/blocksuite/presets/ai';
+import { AIProvider } from '@affine/core/blocksuite/presets';
 import type { OAuthProviderType } from '@affine/graphql';
 import { track } from '@affine/track';
 import { OnEvent, Service } from '@toeverything/infra';