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/AiSettingsDialog.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { onMount } from "svelte";
+  import { onDestroy, onMount } from "svelte";
   import { AlertCircle, Bot, Check, Cpu, Eye, EyeOff, Gauge, Globe, Key, LoaderCircle, Sparkles, X, Zap } from "@lucide/svelte";
   import { credDelete, credLoad, credSave } from "../git";
   import type { AiSettings, CommitAiLocalProfile, CommitAiProvider, LocalModelOption } from "../types";
@@ -36,6 +36,7 @@
   let loadingKeys = $state(true);
   let saving = $state(false);
   let error = $state("");
+  let errorHideTimer: ReturnType<typeof setTimeout> | undefined;
 
   $effect(() => {
     provider = settings.provider;
@@ -47,6 +48,16 @@
     customModel = settings.customModel;
   });
 
+  $effect(() => {
+    if (errorHideTimer) clearTimeout(errorHideTimer);
+    const currentError = error;
+    if (currentError) {
+      errorHideTimer = setTimeout(() => {
+        if (error === currentError) error = "";
+      }, 6000);
+    }
+  });
+
   onMount(() => {
     (async () => {
       try {
@@ -66,6 +77,10 @@
     })();
   });
 
+  onDestroy(() => {
+    if (errorHideTimer) clearTimeout(errorHideTimer);
+  });
+
   async function persistKey(target: CloudProvider, value: string) {
     const key = CRED_KEYS[target];
     const trimmed = value.trim();