chore: bump version to v3.9.1

- Update version in package.json, tauri.conf.json, Cargo.toml
- Update version badges and current version in README files
- Add v3.9.1 changelog entry with bug fixes and improvements

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "CC Switch",
-  "version": "3.9.0",
+  "version": "3.9.1",
   "identifier": "com.ccswitch.desktop",
   "build": {
     "frontendDist": "../dist",