refactor(ci): standardize Tauri signing key variable and update pubkey

- Update CI workflow to use TAURI_SIGNING_PRIVATE_KEY consistently
- Simplify key handling logic and add password support
- Update pubkey in tauri.conf.json to match new signing key

src-tauri/tauri.conf.json

@@ -42,7 +42,7 @@
   ,
   "plugins": {
     "updater": {
-      "pubkey": "RWQrfsDuoYVMiAZTIXmxysD1OTMhVzeirvQ2xU78mhvobMthsQaQyr3m",
+      "pubkey": "dW50cnVzdGVkIGNvbW1lbnQ6IG1pbmlzaWduIHB1YmxpYyBrZXk6IEM4MDI4QzlBNTczOTI4RTMKUldUaktEbFhtb3dDeUM5US9kT0FmdGR5Ti9vQzcwa2dTMlpibDVDUmQ2M0VGTzVOWnd0SGpFVlEK",
       "endpoints": [
         "https://github.com/jasonyoung/cc-switch/releases/latest/download/latest.json"
       ]