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/BranchPanel.svelte

@@ -584,7 +584,7 @@
                   style={`--branch-indent: ${row.depth * 16}px;`}
                   type="button"
                   onclick={() => toggleBranchFolder(row.id)}
-                  oncontextmenu={row.depth > 0 ? (event) => openFolderContextMenu(event, row) : undefined}
+                  oncontextmenu={(event) => openFolderContextMenu(event, row)}
                   aria-expanded={isBranchFolderOpen(row.id)}
                   title={`${row.name} (${row.branchCount})`}
                 >