chore: bump version to 3.9.0-beta.1

- Update version in package.json, Cargo.toml, tauri.conf.json
- Add CHANGELOG entry for v3.9.0-beta.1 with:
  - Local Proxy Server feature
  - Auto Failover with circuit breaker
  - Skills multi-app support
  - Provider icon colors
  - 25+ bug fixes
- Add proxy feature guide documentation (Chinese)

package.json

@@ -1,6 +1,6 @@
 {
   "name": "cc-switch",
-  "version": "3.8.2",
+  "version": "3.9.0-beta.1",
   "description": "All-in-One Assistant for Claude Code, Codex & Gemini CLI",
   "scripts": {
     "dev": "pnpm tauri dev",