ci(release): build macos release artifact

[jorge-huxley]

Summary

Adds a macOS build to the tag-driven release workflow so GitHub Releases publish Windows, macOS, and Android artifacts from the same version tag.

Changes

• Add a macos release job on macos-latest that stamps version and Dropbox app key metadata, runs uv run flet build macos, zips the generated .app, and uploads it as a release artifact.
• Include the macOS artifact in the final release job dependencies.
• Document macOS downloads, Keychain storage, first-run Gatekeeper behavior, and local macOS build commands in the README.

Verification

• Tests pass (uv run pytest -q)
• Workflow YAML parses with PyYAML
• Diff has no whitespace errors (git diff --check)
• macOS release build tested by a user and confirmed working

Notes / follow-ups

• Release builds are not notarized yet, so macOS may require right-click → Open on first launch.

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.