fix(ui): add ARIA labels to booking calendar date cells, weekday headers, and time slot buttons

[LubaKaper]

Issue Summary

The booking calendar contains multiple missing ARIA labels on interactive elements, making the core booking flow
inaccessible to screen reader users. Weekday headers announce nothing, date cells announce only a bare number with no
month or availability context, and time slot buttons announce only a time string with no booking context. These are
the primary interactive elements in the booking flow — if they are inaccessible, the entire booking experience is
inaccessible to blind users.

This is a scoped fix for a subset of the accessibility problems documented in #26416.

Steps to Reproduce

1. Open any public booking page (e.g. /username/event-type)
2. Enable VoiceOver (Mac: Cmd+F5) or open DevTools → Accessibility panel
3. Navigate to the calendar weekday headers ("Mo", "Tu", etc.)
4. Navigate to a date cell — available and disabled
5. Select an available date and navigate to the time slot buttons

Actual Results

Element	Screen reader output	Problem
Weekday headers	Name: "" — complete silence	Role: generic, no ARIA attributes
Date cells	Name: "4" — bare number only	No month or year context
Disabled date cells	Name: "4" — no availability context	User cannot tell why the date is unselectable
Time slot buttons	Name: "1:30pm" — time string only	No booking context, no availability state

Expected Results

Element	Expected screen reader output
Weekday headers	Name: "Monday", Name: "Tuesday", etc.
Date cells	Name: "April 4, 2026"
Disabled date cells	Name: "April 4, 2026, unavailable"
Time slot buttons	Name: "Book 1:30 PM"

Technical details

• Screen reader: VoiceOver (Mac)
• Browser: Chrome
• Verified via DevTools → Accessibility panel on each element
• Files affected:
  • packages/ui/components/form/date-range-picker/Calendar.tsx
  • packages/ui/components/form/datepicker/DatePicker.tsx
  • Booker time slot component

Evidence

Verified using DevTools Accessibility panel. Screenshots attached showing Name: "" on weekday headers, Name: "4"
on date cells, and Name: "1:30pm" on time slot buttons.

[Praneeth9nitin]

Hello I'd like to work on this issue

[LubaKaper]

@Praneeth9nitin Hi! We're already working on this , branch is set up and first commits are in. Planning to open a draft PR
shortly.t