chore: finalize production metadata

[jjoonleo]

Summary

Closes #396.

• replace scaffold app descriptions and web title with production OnTime metadata
• configure Android release signing through local key.properties or ONTIME_ANDROID_* environment variables
• add release checklist and Android signing setup instructions for Play App Signing

Validation

• flutter analyze
• flutter test
• flutter build web
• flutter build apk --debug
• flutter build apk --release without signing config fails with the intended signing message
• flutter build apk --release with a temporary env-backed test keystore succeeds

[github-actions]

Visit the preview URL for this PR (updated for commit 187f9e3):

https://ontime-c63f1--pr402-codexd-issue-396-pro-b2o9le8b.web.app

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[github-actions]

Visit the preview URL for this PR (updated for commit 187f9e3):

https://on-time-front-widgetbook--pr402-codexd-issue-396-pro-uieomhrl.web.app

_{(expires Thu, 14 May 2026 08:36:22 GMT)}

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