Enable Windows update checks

[WangEdgar]

What changed

• Enables the tray 检查更新 flow on Windows instead of disabling the menu path.
• Points stable and beta updater metadata to WangEdgar/DashiTaskboard.
• Adds Windows NSIS updater artifacts and signatures to the release workflow and latest.json.
• Adds a Windows updater verifier and updates release-path assertions.
• Updates README release download links to the maintained repository.

Verified

• node --test test/launcher-release.test.mjs
• Parsed .github/workflows/release-macos.yml and src-tauri/tauri.conf.json
• git diff --check
• cargo check --manifest-path src-tauri/Cargo.toml --target x86_64-pc-windows-msvc after preparing Tauri resources

Notes

This has not been installed into the local desktop app yet. Windows update installation still depends on publishing a release that includes the new Windows updater zip, signature, and merged latest.json.