fix(ui): Calendar view respects user-selected timezone

[anmolxlight]

Problem

The Calendar view in the Airflow UI always shows Dag runs in UTC, ignoring the user's selected timezone from the TimezoneContext. Runs are grouped by their raw UTC date/hour via string slicing, and the calendar grid bounds are computed using the browser-local timezone.

Root Cause

The Calendar feature tree had no awareness of useTimezone() / TimezoneContext:

1. API queries — The date range sent to the API used selectedDate directly without timezone conversion
2. Run grouping — createDailyDataMap and createHourlyDataMap used run.date.slice(0, 10) / run.date.slice(0, 13) to extract date/hour, ignoring the user's timezone
3. Grid construction — generateDailyCalendarData and generateHourlyCalendarData used dayjs().year(...) instead of dayjs().tz(timezone).year(...), computing grid bounds in browser-local time

Changes

• calendarUtils.ts — Added dayjs/plugin/timezone and dayjs/plugin/utc imports. Threaded a timezone: string parameter through all exported/internal functions. Replaced run.date.slice(0, 10) / run.date.slice(0, 13) with dayjs(run.date).tz(timezone).format(...). Grid construction now uses dayjs().tz(timezone) instead of bare dayjs().

• Calendar.tsx — Imported useTimezone, added dayjs tz/utc plugins. Gets selectedTimezone from context. Computes startDate/endDate in the selected timezone via selectedDate.tz(selectedTimezone, true), then converts to UTC with .utc().format(...) for API queries. Passes timezone prop to view components and createCalendarScale.

• DailyCalendarView.tsx — Accepts timezone prop, passes it to generateDailyCalendarData.

• HourlyCalendarView.tsx — Accepts timezone prop, passes it to generateHourlyCalendarData.

• calendarUtils.test.ts — All calculateDataBounds and createCalendarScale calls updated with "UTC" as the 4th argument.

Fixes #67477