Add Google Calendar integration

- Add GoogleCalendarClient for fetching upcoming events
- Add GoogleCalendarAPI interface and CalendarEvent model
- Add config for GOOGLE_CREDENTIALS_FILE and GOOGLE_CALENDAR_ID
- Display events in Planning tab with date/time formatting
- Update handlers and tests to support optional calendar client

Config env vars:
- GOOGLE_CREDENTIALS_FILE: Path to service account JSON
- GOOGLE_CALENDAR_ID: Calendar ID (defaults to "primary")

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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