refactor(phase3): remove OpenClaw compatibility layer

Phase 3 Configuration Migration completed:
- Remove OpenClaw backend type selector from General.tsx
- Update default workspace path from ~/.openclaw to ~/.openfang
- Update default port from 18789 to 50051
- Archive openclaw.default.json to docs/archive/openclaw-legacy/
- Remove OpenClaw migration UI from Workspace settings
- Update About.tsx to reference OpenFang
- Clean up gateway-client.ts comments

All OpenClaw compatibility code removed. ZCLAW now exclusively
targets OpenFang (Rust Agent OS) backend.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src/components/Settings/About.tsx

@@ -34,10 +34,10 @@ export function About() {
       </div>
 
       <div className="mt-12 text-center text-xs text-gray-400">
-        2026 ZCLAW | Powered by OpenClaw
+        2026 ZCLAW | Powered by OpenFang
       </div>
       <div className="text-center text-xs text-gray-400 space-y-1">
-        <p>基于 OpenClaw 开源框架定制</p>
+        <p>基于 OpenFang Rust Agent OS 构建</p>
         <div className="flex justify-center gap-4 mt-3">
           <a href="#" className="text-orange-500 hover:text-orange-600">隐私政策</a>
           <a href="#" className="text-orange-500 hover:text-orange-600">用户协议</a>