The New Branch dialog previously ran detectDefaultBranch and
listGitBranches sequentially in onMount, causing a visible pause before
the dialog content appeared.

- Use Promise.allSettled to fetch both in parallel
- Add a loading state with a spinner shown while data is being fetched
- Add .loading-state CSS for centered spinner layout

The New Branch dialog still blocked the UI because:

1. list_git_branches and detect_default_branch_cmd were synchronous
   Tauri commands (fn, not async fn), which run on the main thread
   and freeze the entire window until they return.

2. list_branches() called `git remote prune origin` on every open —
   a network operation that adds significant latency to what should
   be a fast local operation.

Changes:
- Make list_git_branches and detect_default_branch_cmd async so they
  run on Tauri's async thread pool instead of the main thread
- Extract `git remote prune origin` into a separate prune_remote()
  function and expose it as a new prune_remote_refs Tauri command
- Call pruneRemoteRefs as fire-and-forget in the background after the
  dialog has loaded, then silently refresh the branch list when done
- Errors from pruning are silently ignored (best-effort, e.g. no
  network or no remote configured)