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/lib/gateway-client.ts

@@ -5,11 +5,11 @@
  * in the Tauri React frontend. Uses native browser WebSocket API.
  * Supports Ed25519 device authentication + JWT.
  *
- * OpenFang vs OpenClaw:
- * - Port: 4200 (was 18789)
+ * OpenFang Configuration:
+ * - Port: 50051
  * - WebSocket path: /ws
- * - REST API: http://127.0.0.1:4200/api/*
- * - Config format: TOML (was YAML/JSON)
+ * - REST API: http://127.0.0.1:50051/api/*
+ * - Config format: TOML
  */
 
 import nacl from 'tweetnacl';