Add cross-platform release CI

[flandyw]

Adds an automated release workflow for the desktop app.

Behavior:

• builds Windows x64, macOS, and Linux x64 release packages
• uploads packaged builds as GitHub Actions artifacts
• validates that a release tag such as v1.0.0 matches the version in pubspec.yaml
• on v* tags, downloads all platform packages, generates SHA256SUMS.txt, and creates a GitHub Release with generated release notes
• supports manual workflow runs that build artifacts without publishing a release
• runs the cross-platform build matrix on this PR because the workflow file itself changed
• fails packaging if an expected build bundle is missing

Release usage after merge:

1. bump version: in pubspec.yaml
2. merge the version change to master
3. create/push the matching tag, e.g. v1.1.0
4. Release CI builds all three platforms and publishes the GitHub Release automatically.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 76e5b70 and e1bff8a.

📒 Files selected for processing (1)

• .github/workflows/release.yml

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