feat(git): support credentials for remote branch rename

Allow remote branch renames to be performed with optional credentials so
operations against protected remotes succeed when authentication is needed.
The backend accepts username/password and uses an authenticated push path
when provided, while the frontend prompts for and reuses stored credentials.

- Add optional username/password to rename RPC and use authenticated push
- Wire UI to queue rename, open credential dialog, and execute rename
- Extend credential dialog and handling to include the rename action

src/lib/components/CredentialDialog.svelte

@@ -15,7 +15,7 @@
   } from "@lucide/svelte";
 
   interface Props {
-    action: "push" | "pull" | "fetch" | "clone";
+    action: "push" | "pull" | "fetch" | "clone" | "rename";
     error: string;
     isBusy: boolean;
     initialUsername?: string;
@@ -47,17 +47,19 @@
     password.trim().length > 0 &&
     username.trim().length > 0,
   );
-  let actionLabel = $derived(action === "push" ? "Push" : action === "fetch" ? "Fetch" : action === "clone" ? "Clone" : "Pull");
+  let actionLabel = $derived(action === "push" ? "Push" : action === "fetch" ? "Fetch" : action === "clone" ? "Clone" : action === "rename" ? "Rename" : "Pull");
   let actionTitle = $derived(
     action === "push"
       ? "Authenticate push"
-      : action === "fetch"
+      : action === "rename"
+        ? "Authenticate remote rename"
+        : action === "fetch"
         ? "Authenticate fetch"
         : action === "clone"
           ? "Authenticate clone"
           : "Authenticate pull",
   );
-  let actionHint = $derived(action === "push"
+  let actionHint = $derived(action === "push" || action === "rename"
     ? "The remote needs write access. Use a password or a token with the appropriate repository permissions."
     : action === "clone"
       ? "The repository needs access before it can be cloned. Use your Git credentials or a personal access token."
@@ -78,7 +80,7 @@
     <div class="cred-hero">
       <div class="cred-hero-top">
         <div class="cred-hero-icon">
-          {#if action === "push"}
+          {#if action === "push" || action === "rename"}
             <Upload size={27} aria-hidden="true" />
           {:else}
             <Download size={27} aria-hidden="true" />