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/lib/components/CloneRepositoryDialog.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+  import { onDestroy } from "svelte";
   import { open as openDialog } from "@tauri-apps/plugin-dialog";
   import { Download, FolderOpen, LoaderCircle, X } from "@lucide/svelte";
 
@@ -22,6 +23,8 @@
   let directoryNameEdited = $state(false);
   let directoryAutoName = $state("");
   let browseError = $state("");
+  let visibleError = $state("");
+  let errorHideTimer: ReturnType<typeof setTimeout> | undefined;
 
   let directorySuggestion = $derived(directoryNameFromRemoteUrl(remoteUrl));
   let canSubmit = $derived(
@@ -29,7 +32,21 @@
     remoteUrl.trim().length > 0 &&
     parentPath.trim().length > 0,
   );
-  let visibleError = $derived(error || browseError);
+
+  $effect(() => {
+    const nextError = error || browseError;
+    if (errorHideTimer) clearTimeout(errorHideTimer);
+    visibleError = nextError;
+    if (nextError) {
+      errorHideTimer = setTimeout(() => {
+        visibleError = "";
+      }, 6000);
+    }
+  });
+
+  onDestroy(() => {
+    if (errorHideTimer) clearTimeout(errorHideTimer);
+  });
 
   function directoryNameFromRemoteUrl(url: string): string {
     const trimmed = url.trim().split(/[?#]/, 1)[0]?.replace(/[\\/]+$/g, "") ?? "";