fix: handle unborn HEAD branch creation

- Create orphan branches when HEAD does not exist yet\n- Use current branch as the default branch base in the toolbar\n- Keep push disabled for remote-less repos\n\nQuality gates: vitest repositories-settings-tab (passed); api pytest blocked by missing fastapi in environment

apps/api/src/utils/git_control.py

@@ -121,6 +121,13 @@ def create_branch(repo_path: str, name: str, base_branch: str = "HEAD") -> None:
     Raises:
         RuntimeError: If branch creation fails
     """
+    if base_branch == "HEAD":
+        try:
+            _run_git_command(repo_path, "rev-parse", "--verify", "HEAD")
+        except RuntimeError:
+            _run_git_command(repo_path, "checkout", "--orphan", name)
+            return
+
     _run_git_command(repo_path, "branch", name, base_branch)