Introduce a non-blocking update check and in-app updater.

Key changes:
- deeplabcut/gui/utils.py: add check_pypi_version, is_latest_deeplabcut_version/is_latest_plugin_version, and UpdateCheckWorker (Qt QObject) to fetch PyPI info with timeouts and packaging-aware version comparison; handle network errors gracefully.
- deeplabcut/gui/window.py: replace the blocking _check_for_updates flow with a Qt-threaded update check (schedule_update_check, _start_update_check) that uses UpdateCheckWorker; add UI handling for results, prompt to update and run pip installs via QProcess, and cancel/cleanup on exit. Removed previous blocking imports/usages (subprocess, threading, call_with_timeout).
- deeplabcut/utils/multiprocessing.py: mark call_with_timeout as deprecated for update checks (note only), since it's no longer used for update checking.

Why: make update checking robust and non-blocking, avoid freezing the GUI, support plugin/version checks, provide an integrated, user-friendly update flow with proper error handling.
Co-Authored-By: Mikhail Mikhalchuk <[email protected]>

Accumulate and surface output from the pip update QProcess so users get live feedback and the full output after the process finishes. Added a _update_process_output list, initialized when starting/tearing down updates, and a helper _drain_update_process_output() that reads available data, appends it to the buffer, logs the latest non-empty line and shows it in the status bar. The finished handler now drains remaining output, builds the final output by joining the buffer, and resets the buffer and status message. Also added a clarifying comment in utils.py about making the plugin check optional in the future.

Reduce the UpdateCheckWorker timeout from 5.0s to 2.0s to make plugin/version checks quicker, and increase the update thread quit wait from 1000ms to 4000ms to give the thread more time to shut down cleanly. Changes applied in deeplabcut/gui/utils.py and deeplabcut/gui/window.py.

Ensure cancellation is handled

Missed deleteLater (qt tries to clean this up)

Replace the old thread/worker based update check with a single QtNetwork-backed UpdateChecker that issues async HTTP requests to PyPI for both deeplabcut and napari-deeplabcut, enforces a QTimer timeout, aggregates results, and supports silent/manual checks and cancellation. Integrate the new checker into MainWindow (use DLC and NAPARI_DLC __version__ imports), simplify scheduling, and connect finished signal to the existing handler. Also add safer shutdown handling (_closing flag), drain update process output, and minor UI/version label updates.

Replace singleShot scheduling with a dedicated QTimer to manage update checks. Adds _update_check_timer and _scheduled_update_check_silent to schedule, supersede, and run delayed checks via check_for_updates(..., delay_ms). Wire up startup and menu actions to the new API and stop the timer during shutdown. This prevents overlapping checks, allows cancelation on close, and centralizes delayed-check logic.

Add "packaging>=26.0" to pyproject.toml dependencies and update uv.lock to include the new requirement.