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/HistoryPanel.svelte

@@ -316,12 +316,6 @@
     }
   }
 
-  function handleBranchDialogBackdropClick(event: MouseEvent) {
-    if (event.target === event.currentTarget) {
-      closeBranchDialog();
-    }
-  }
-
   function localBranchRefs(commit: GitCommit): string[] {
     const seen = new Set<string>();
     const labels: string[] = [];
@@ -569,7 +563,7 @@
 </section>
 
 {#if branchDialogOpen}
-  <div class="branch-filter-backdrop" role="presentation" onclick={handleBranchDialogBackdropClick}>
+  <div class="branch-filter-backdrop" role="presentation">
     <div
       class="branch-filter-dialog"
       role="dialog"