feat(git): rename remote branches atomically via push

Adds a new command to rename remote branches and update tracking refs.
Renaming remote refs is performed atomically using a single push.
The push creates the new remote ref and deletes the old if it succeeds.
The frontend now invokes remote-rename when needed and shows labels.

- Atomic remote rename via push with create/ref and delete
- Frontend supports remote branch renames from the branch panel
- Compare UI now shows labels for remote refs in results

src/lib/RepoToolbar.svelte

@@ -151,8 +151,8 @@
       class="repo-action"
       onclick={onCompare}
       disabled={!hasRepository || isBusy}
-      title={isGerman ? "Commits vergleichen" : "Compare commits"}
-      aria-label={isGerman ? "Commits vergleichen" : "Compare commits"}
+      title={isGerman ? "Branches oder Commits vergleichen" : "Compare branches or commits"}
+      aria-label={isGerman ? "Branches oder Commits vergleichen" : "Compare branches or commits"}
     >
       <GitCompare size={15} aria-hidden="true" />
       <span class="repo-action-label">{isGerman ? "Vergleichen" : "Compare"}</span>