feat(remote): support authenticated deletion of remote branches

Add support for authenticated deletion of remote branches and folders.
The backend adds a batch delete command that queries the remote,
performs an atomic push --delete when possible with a fallback, and
prunes stale tracking refs; it accepts optional credentials. The
frontend and JS API integrate credential handling and prompt users when
authentication is required.

- New batch deletion command with optional username/password for auth.
- Uses ls-remote to scope deletions, tries --atomic then falls back.
- Frontend updates: credential dialog/action, pendingRemoteDelete state.

src/lib/components/CredentialDialog.svelte

@@ -15,7 +15,7 @@
   } from "@lucide/svelte";
 
   interface Props {
-    action: "push" | "pull" | "fetch" | "clone" | "rename";
+    action: "push" | "pull" | "fetch" | "clone" | "rename" | "delete";
     error: string;
     isBusy: boolean;
     initialUsername?: string;
@@ -47,19 +47,21 @@
     password.trim().length > 0 &&
     username.trim().length > 0,
   );
-  let actionLabel = $derived(action === "push" ? "Push" : action === "fetch" ? "Fetch" : action === "clone" ? "Clone" : action === "rename" ? "Rename" : "Pull");
+  let actionLabel = $derived(action === "push" ? "Push" : action === "fetch" ? "Fetch" : action === "clone" ? "Clone" : action === "rename" ? "Rename" : action === "delete" ? "Delete" : "Pull");
   let actionTitle = $derived(
     action === "push"
       ? "Authenticate push"
       : action === "rename"
         ? "Authenticate remote rename"
-        : action === "fetch"
-        ? "Authenticate fetch"
-        : action === "clone"
-          ? "Authenticate clone"
-          : "Authenticate pull",
+        : action === "delete"
+          ? "Authenticate remote deletion"
+          : action === "fetch"
+            ? "Authenticate fetch"
+            : action === "clone"
+              ? "Authenticate clone"
+              : "Authenticate pull",
   );
-  let actionHint = $derived(action === "push" || action === "rename"
+  let actionHint = $derived(action === "push" || action === "rename" || action === "delete"
     ? "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."
@@ -80,7 +82,7 @@
     <div class="cred-hero">
       <div class="cred-hero-top">
         <div class="cred-hero-icon">
-          {#if action === "push" || action === "rename"}
+          {#if action === "push" || action === "rename" || action === "delete"}
             <Upload size={27} aria-hidden="true" />
           {:else}
             <Download size={27} aria-hidden="true" />