simplify calendar into a month-first agenda

[deancochran]

Summary

• replace the dual-mode calendar with a month-first flow and a selected-day agenda panel
• simplify calendar state and query-window logic by removing unused day-mode paths and components
• preserve event preview, create, and move flows while updating mobile tests and adding a spec

Testing

• pnpm test:jest:file calendar-screen.jest.test.tsx
• pnpm test:jest:file CalendarPaging.jest.test.tsx
• pnpm check-types
• pnpm lint

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