feat: notify when a new release is available

[hieshima]

Summary

• Check GitHub Releases for a newer Mouser version without blocking app startup.
• Add a manual "Check for updates" path and a user-facing notification that opens the GitHub release page.
• Add a setting to disable automatic update checks while keeping manual checks available.
• Add tests for version comparison, network failures, malformed responses, disabled automatic checks, and manual checks.

Partially Addresses

• [Feature] Auto Update #152: adds update discovery and a direct path to the latest release page, but does not perform automatic download, replacement, or installation.

Follow-up Scope

• Automatic download-and-install updates remain a later production-grade updater track.
• feat: auto update feature with linux deb support #117 stays open for the broader auto-install / packaging-distribution work.

Validation

• Local tests:
  • python -m pytest tests/test_updater.py tests/test_config.py tests/test_backend.py
  • python -m pytest
  • git diff --check
• Manual artifact validation should verify that startup remains responsive and the update notification UI does not block normal mouse configuration.

Notes

• This PR does not download artifacts.
• This PR does not run installers, package managers, shell replacement scripts, or privilege-escalation helpers.
• This PR does not change the Release workflow or introduce latest.json.