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/AiSettingsDialog.svelte

@@ -126,7 +126,6 @@
 <div
   class="dialog-backdrop"
   role="presentation"
-  onclick={(e) => { if (e.target === e.currentTarget) onClose(); }}
 >
   <div class="dialog ai-settings-dialog" role="dialog" aria-modal="true" aria-label="AI settings" tabindex="-1">
     <header class="dialog-header">