feat(clone): add repository cloning flow to UI and backend

This introduces a new Tauri command to clone a remote repository into a
validated destination folder, then return the full repository bundle for
immediate rendering. The Svelte app gains a clone dialog and a new action
in repository management, wiring the cloned bundle into existing refresh
helpers. Dialog backdrops were also adjusted to rely on explicit close
controls.

- Add clone_repository command and core cloning logic in Rust
- Create CloneRepositoryDialog and integrate it into App.svelte
- Improve clone-related styling and tighten dialog backdrop behavior

src/lib/components/BranchDeleteConfirmDialog.svelte

@@ -20,13 +20,9 @@
 
   let title = $derived(force ? "Force delete branch?" : "Delete branch?");
 
-  function closeFromBackdrop(event: MouseEvent) {
-    if (isBusy || event.target !== event.currentTarget) return;
-    onClose();
-  }
 </script>
 
-<div class="dialog-backdrop" role="presentation" onclick={closeFromBackdrop}>
+<div class="dialog-backdrop" role="presentation">
   <div class="dialog branch-delete-dialog" role="dialog" aria-modal="true" aria-label={title}>
     <header class="dialog-header">
       <div>