Fix: make widget.loading thread-safe (closes #5108)

[jtvhd6]

Closes #5108.

_watch_loading now checks whether it's running on the app's own
thread before mutating the widget's DOM. If loading is set from a
worker thread, the update is routed through App.call_from_thread
instead of touching the UI directly.

Testing

• Added test_loading_set_from_worker_thread in tests/test_loading.py,
  which sets .loading = True from a real background thread via
  run_worker(thread=True) and asserts it doesn't raise and updates
  the reactive correctly.
• poetry run pytest tests/ -k loading -q — 20 passed.
• poetry run ruff check — all checks passed.
• poetry run mypy src/textual/widget.py — confirmed 12 pre-existing
  errors are unrelated to this change (same errors present on main).