feat: Make semantic search optional and disabled by default

- Add SemanticSearchConfig interface with enabled toggle to siteConfig.ts
- Default semantic search to disabled (enabled: false) to avoid blocking forks without OPENAI_API_KEY
- Update SearchModal.tsx to conditionally show mode toggle based on config
- Update sync-posts.ts to skip embedding generation when disabled
- Add semantic search toggle to Dashboard config generator
- Update FORK_CONFIG.md with Semantic Search Configuration section
- Update fork-config.json.example with semanticSearch option
- Update docs-semantic-search.md with enable/disable instructions
- Update changelog and documentation

When disabled (default):
- Search modal shows only keyword search (no mode toggle)
- Embedding generation skipped during sync
- No OpenAI API key required

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

fork-config.json.example

@@ -179,6 +179,9 @@
   "dashboard": {
     "enabled": true,
     "requireAuth": false
+  },
+  "semanticSearch": {
+    "enabled": false
   }
 }