chore(release): prepare v3.9.0 stable release

- Bump version from 3.9.0-3 to 3.9.0 across all config files
- Add comprehensive release notes in English, Chinese, and Japanese
- Update CHANGELOG with v3.9.0 stable and v3.9.0-2 entries
- Update README badges and release note links to v3.9.0

src-tauri/Cargo.lock

@@ -701,7 +701,7 @@ dependencies = [
 
 [[package]]
 name = "cc-switch"
-version = "3.9.0-3"
+version = "3.9.0"
 dependencies = [
  "anyhow",
  "async-stream",

src-tauri/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cc-switch"
-version = "3.9.0-3"
+version = "3.9.0"
 description = "All-in-One Assistant for Claude Code, Codex & Gemini CLI"
 authors = ["Jason Young"]
 license = "MIT"

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "CC Switch",
-  "version": "3.9.0-3",
+  "version": "3.9.0",
   "identifier": "com.ccswitch.desktop",
   "build": {
     "frontendDist": "../dist",