fix(web): stabilize up next card

[tyler-dane]

Summary

• keep the Up next card mounted when no visible future event remains, showing a quiet All clear state
• give both card states the same minimum height so calendar toggles do not shift
• cover hiding the only upcoming event calendar

Validation

• bun test:web packages/web/src/components/Sidebar/UpNextCard/UpNextCard.test.tsx
• bun run type-check
• bun run lint
• bun test:web (1,549 passing)
• bun test:a11y
• manual browser pass at http://localhost:9092/day/2026-08-01: created an upcoming event, hid its calendar, verified All clear rendered in place, the calendar row retained focus and position, and browser console had no warnings/errors.

Review

• Simplification considered: no extraction or new state needed; the button and passive state intentionally have distinct semantics while sharing the fixed height.
• Independent review: no actionable findings.