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/app.css

@@ -2363,6 +2363,54 @@
     max-height: calc(100vh - 32px);
     overflow: auto;
   }
+  .clone-repository-dialog {
+    display: block;
+    width: min(620px, calc(100vw - 32px));
+    height: auto;
+    max-height: calc(100vh - 32px);
+    overflow: auto;
+  }
+  .clone-dialog-form {
+    display: grid;
+    gap: 14px;
+    padding: 16px;
+  }
+  .clone-dialog-field {
+    display: grid;
+    gap: 5px;
+    min-width: 0;
+  }
+  .clone-dialog-field > span {
+    overflow: hidden;
+    color: var(--color-ink-faint);
+    font-size: 10.5px;
+    font-weight: 800;
+    text-transform: uppercase;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+    letter-spacing: 0.04em;
+  }
+  .clone-dialog-path-field {
+    display: grid;
+    grid-template-columns: minmax(0, 1fr) auto;
+    gap: 8px;
+    min-width: 0;
+  }
+  .clone-dialog-error {
+    padding: 9px 10px;
+    border: 1px solid rgba(232,96,90,0.3);
+    border-radius: 7px;
+    color: #f09090;
+    background: rgba(232,96,90,0.08);
+    font-size: 12.5px;
+    line-height: 1.45;
+  }
+  .clone-dialog-actions {
+    display: flex;
+    justify-content: flex-end;
+    gap: 8px;
+    padding-top: 2px;
+  }
   .ai-settings-form {
     display: flex;
     flex-direction: column;
@@ -3779,6 +3827,12 @@
   .repo-tabbar    { grid-template-columns: minmax(0, 1fr) auto; }
   .repo-tab.management { min-width: 0; }
   .repo-tabs-scroll { grid-column: 1 / -1; order: 2; border-top: 1px solid var(--color-border-subtle); }
+  .repo-management-head,
+  .repo-management-tools { align-items: stretch; flex-direction: column; }
+  .repo-management-actions { justify-content: flex-start; }
+  .clone-dialog-path-field { grid-template-columns: minmax(0, 1fr); }
+  .clone-dialog-actions { flex-direction: column-reverse; }
+  .clone-dialog-actions button { width: 100%; }
   .repo-row-main  { grid-template-columns: minmax(0, 1fr); gap: 2px; align-content: center; padding-left: 12px; }
   .repo-row       { min-height: 58px; }
   .repo-row-icon  { min-height: 58px; }