feat(clone): support credentialed git clone with askpass

Cloning now accepts optional username/password and passes them to the
Rust git layer via GIT_ASKPASS, avoiding interactive prompts. The UI
detects authentication failures, opens the credential dialog for clone,
and auto-hides error messages to keep the flow smooth.

- Add username/password support to clone_repository and git.ts
- Detect auth failures and route users to the clone credential dialog
- Improve clone UX with a dedicated loading overlay and timed errors

src-tauri/tauri.conf.json

@@ -29,7 +29,7 @@
   "bundle": {
     "active": true,
     "targets": ["nsis"],
-    "icon": ["icons/icon.ico"],
+    "icon": ["icons/icon.png", "icons/icon.ico"],
     "createUpdaterArtifacts": true,
     "windows": {
       "nsis": {