Complete Dante 2.0.12 fixes

[blackbartblues]

Summary

Completes the unreleased Dante 2.0.12 patch with the remaining fixes.

• add native Google Calendar API transport and clearer reauthorization handling
• allow changing a nonempty shopping list owner/visibility when its catalog scope is unchanged
• keep the existing safeguard that rejects moving a nonempty list between Home and Work scopes
• use a synthetic profile in calendar tests so public-readiness validation remains clean

Validation

• backend: 385 tests passed
• version consistency passed
• public-readiness self-test passed
• public-readiness repository scan passed

This PR is intentionally left as a draft; do not publish 2.0.12 yet.