feat(daily): add Calendar widget showing today's Google Calendar events

[AggManik]

• New native widget CalendarWidget renders today's events sorted by start time, with all-day events at the top
• New API route GET /api/v1/calendar/today-events fetches events via the google-calendar integration using the user's local timezone
• Registers "calendar" in NATIVE_WIDGETS so users can add it via the widget picker in the Daily section
• Updates google-calendar list_events to return structured JSON instead of a formatted string so the route can parse event objects cleanly

[AggManik]

Patched this PR to avoid throwing when Google Calendar isn’t connected.

Changes (commit 74b7da0 on feature/calendar-widget-plan):

• Guard missing authentication.workspaceId → return { events: [], connected: false }
• Wrap IntegrationLoader.getConnectedIntegrationAccounts(...) in try/catch → on error return { events: [], connected: false }

Net: calendar widget renders the empty/not-connected state instead of 500ing.