Polish calendar workspace interaction states

[Errordog2]

Summary

• Polish the existing Calendar Workspace empty states so hidden calendars, empty agenda days, and empty months explain the next action clearly.
• Add pressed/focus treatment for calendar toggles, visible disabled copy for missing custom-calendar names, and a short copying/copied state with failure handling for event details.
• Keep the work inside the existing calendar module and preserve the existing event/calendar behavior.

Closes #921

Before/after note

• Before: empty calendar views reused generic copy, invisible calendars were harder to distinguish, and the copy-details action had no transient feedback.
• After: the workspace explains why the view is empty, toggles expose pressed/focus states, disabled actions tell users what is missing, and copy details reports copying/copied/failure states.

Validation

• npx prettier --check src/features/calendar/components/CalendarWorkspace.tsx src/features/calendar/styles.css
• git diff --check
• sensitive scan for account/payment/private-key markers returned no matches
• npx tsc --noEmit could not run locally because this checkout has no installed TypeScript compiler and npm attempted to fetch the deprecated tsc package
• npm run lint -- src/features/calendar/components/CalendarWorkspace.tsx src/features/calendar/styles.css could not start locally because eslint is not installed in this checkout