feat: add verified Windows update installation

[hieshima]

Summary

• Add release metadata generation and verification for Mouser updates, including version, build number, expiry, platform assets, file sizes, and SHA256 checksums.
• Add a Windows update preparation flow behind a maintainer validation gate. When enabled for validation, Mouser downloads the selected Windows asset, verifies it, rejects unsafe archives, stages the replacement, and hands off to a helper outside the install folder after Mouser exits.
• Add progress, cancellation, startup result handling, cleanup of stale update files, and localized user-facing update errors.
• Add maintainer test endpoint overrides so release discovery and update metadata can be validated before a public release exists: MOUSER_UPDATE_LATEST_RELEASE_URL, MOUSER_UPDATE_MANIFEST_URL, and MOUSER_ENABLE_UPDATE_INSTALL.
• Keep macOS and Linux on the manual install path. Mouser does not replace .app bundles, run package managers, or self-update Linux archives in this PR.

Addresses

• Partially addresses [Feature] Auto Update #152 by adding the Windows install infrastructure. The user-visible Windows install action remains disabled until final GUI Install-click validation passes.

Related

• feat: auto update feature with linux deb support #117 remains the broader cross-platform automatic-install tracking issue.

Validation

• python -m pytest tests/test_updater.py tests/test_update_installer.py tests/test_backend.py tests/test_locale_manager.py
• python -m pytest
• python -m py_compile main_qml.py core/*.py ui/*.py
• QML lint using PySide6 qmllint
• git diff --check
• Windows packaged validation on real Windows 11: the GUI Download path reached the verified/staged ready-to-install state, and the packaged helper/swap/relaunch path updated a test build from 3.7.90 to 3.7.91 while preserving settings and the recovery backup.
• The temporary windows-update-installer CI job was used for draft validation and has been removed from this PR workflow so it does not become permanent CI.

Required Before Enabling Windows Install

• Run one final Windows GUI smoke with the install gate enabled: Check for Updates, Download, Install, relaunch, and confirm settings/profile preservation.
• After that validation passes, a follow-up can enable the user-visible Windows install action by changing the default gate in ui/backend.py.

Notes

• Uses GitHub HTTPS plus SHA256 and size verification. It does not introduce private signing-key custody.
• Does not add macOS automatic app replacement or Linux package/archive self-replacement.
• Does not change release version.