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/components/RenameBranchDialog.svelte

@@ -18,14 +18,29 @@
 
   let name = $state("");
 
+  function remoteName(branch: GitBranchInfo): string {
+    if (!branch.remote) return "";
+    const slash = branch.name.indexOf("/");
+    return slash > 0 ? branch.name.slice(0, slash) : branch.name;
+  }
+
+  function editableName(branch: GitBranchInfo): string {
+    if (!branch.remote) return branch.name;
+    const slash = branch.name.indexOf("/");
+    return slash >= 0 ? branch.name.slice(slash + 1) : branch.name;
+  }
+
+  let originalName = $derived(editableName(branch));
+  let remote = $derived(remoteName(branch));
+
   $effect(() => {
-    name = branch.name;
+    name = originalName;
   });
 
   function submit(event: SubmitEvent) {
     event.preventDefault();
     const value = name.trim();
-    if (!value || value === branch.name) return;
+    if (!value || value === originalName) return;
     onRename(value);
   }
 </script>
@@ -34,10 +49,10 @@
   class="dialog-backdrop"
   role="presentation"
 >
-  <div class="dialog rename-branch-dialog" role="dialog" aria-modal="true" aria-label="Rename branch" tabindex="-1">
+  <div class="dialog rename-branch-dialog" role="dialog" aria-modal="true" aria-label={branch.remote ? "Rename remote branch" : "Rename branch"} tabindex="-1">
     <header class="dialog-header">
       <div>
-        <span class="eyebrow">Rename branch</span>
+        <span class="eyebrow">{branch.remote ? "Rename remote branch" : "Rename branch"}</span>
         <h2 class="mt-0.5 text-ink text-base font-bold leading-tight">{branch.name}</h2>
       </div>
       <button class="dialog-close" type="button" onclick={onClose} title="Close">
@@ -48,27 +63,37 @@
     <form class="rename-branch-form" onsubmit={submit}>
       <label class="new-branch-field">
         <span>Branch name</span>
-        <!-- svelte-ignore a11y_autofocus -->
-        <input
-          bind:value={name}
-          autocomplete="off"
-          spellcheck="false"
-          disabled={isBusy}
-          autofocus
-        />
+        <div class:remote-branch-name-field={branch.remote}>
+          {#if branch.remote}<strong>{remote}/</strong>{/if}
+          <!-- svelte-ignore a11y_autofocus -->
+          <input
+            bind:value={name}
+            autocomplete="off"
+            spellcheck="false"
+            disabled={isBusy}
+            autofocus
+          />
+        </div>
       </label>
 
+      {#if branch.remote}
+        <p class="rename-remote-note">
+          Gitty creates <strong>{remote}/{name.trim() || "new-name"}</strong> and removes
+          <strong>{branch.name}</strong> in one atomic push. The operation stops if the remote changed in the meantime.
+        </p>
+      {/if}
+
       <div class="new-branch-actions">
         <button class="btn-secondary" type="button" onclick={onClose} disabled={isBusy}>
           Cancel
         </button>
-        <button class="btn-primary" type="submit" disabled={isBusy || name.trim().length === 0 || name.trim() === branch.name}>
+        <button class="btn-primary" type="submit" disabled={isBusy || name.trim().length === 0 || name.trim() === originalName}>
           {#if isBusy}
             <LoaderCircle class="spin" size={16} aria-hidden="true" />
           {:else}
             <GitBranch size={16} aria-hidden="true" />
           {/if}
-          Rename
+          {branch.remote ? "Rename on remote" : "Rename"}
         </button>
       </div>
     </form>