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)

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "CC Switch",
-  "version": "3.8.2",
+  "version": "3.9.0-beta.1",
   "identifier": "com.ccswitch.desktop",
   "build": {
     "frontendDist": "../dist",