Limit diary overview to the selected month

[tiye]

Summary

• build the home-page diary overview only for the cursor's selected month
• avoid sending and recursively diffing every historical diary entry after login
• keep the complete diary history in storage and preserve the explicit data-export route
• add regression coverage for month filtering

Context

Production has a much larger diary map than local development. Recollect recursively walks common map entries during client synchronization, so rebuilding an all-history overview can exhaust the Calcit process stack. The calendar only consumes one month at a time, bounding its synchronized overview to roughly 31 entries.

Verification

• calcit test --summary-only (20 passed)
• server --check-only
• server dynamic-method analysis reports zero findings