feat: redesign day picker with expandable calendar, status markers & Playwright E2E testing

- Replace modal calendar with inline expandable calendar widget integrated into the day picker.
- Add status indicator dots (attended/absent/pending) to month calendar grid matching day picker.
- Disable and gray out weekend days in calendar when weekend schedule is turned off.
- Redesign extend bar with integrated handle that aligns with the palette.
- Remove start-to-end timing on Today screen's class time rail, showing only start time.
- Implement Playwright E2E and visual testing suite with multi-screen capture scripts.
- Add cascade DELETE endpoints for subjects and versioned recurring timetable entries.

lib/date.ts

@@ -1,3 +1,17 @@
+// Pre-instantiated Intl formatters hoisted to module scope (per js-hoist-intl best practice)
+const dayHeadingFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'long', month: 'long', day: 'numeric' });
+const monthDayFormatter = new Intl.DateTimeFormat(undefined, { month: 'long', day: 'numeric' });
+const sessionDateFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'short', month: 'short', day: 'numeric' });
+const weekdayNarrowFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'narrow' });
+const weekdayLongFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'long' });
+const monthYearFormatter = new Intl.DateTimeFormat(undefined, { month: 'long', year: 'numeric' });
+const monthShortDayFormatter = new Intl.DateTimeFormat(undefined, { month: 'short', day: 'numeric' });
+const yearFormatter = new Intl.DateTimeFormat(undefined, { year: 'numeric' });
+const timeShortFormatter = new Intl.DateTimeFormat(undefined, { hour: 'numeric', minute: '2-digit' });
+const headerDateFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'long', month: 'short', day: 'numeric' });
+
+const weekdayShortFormatter = new Intl.DateTimeFormat(undefined, { weekday: 'short' });
+
 export const dateKey = (date: Date) => `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`;
 
 export const addDays = (date: Date, amount: number) => {
@@ -10,11 +24,27 @@ export const mondayOfWeek = (date: Date) => addDays(date, -((date.getDay() + 6)
 
 export const dateFromKey = (value: string) => new Date(`${value}T12:00:00`);
 
-export const formatDayHeading = (date: Date) => new Intl.DateTimeFormat(undefined, { weekday: 'long', month: 'long', day: 'numeric' }).format(date);
+export const formatDayHeading = (date: Date) => dayHeadingFormatter.format(date);
 
-export const formatMonthDay = (date: Date) => new Intl.DateTimeFormat(undefined, { month: 'long', day: 'numeric' }).format(date);
+export const formatMonthDay = (date: Date) => monthDayFormatter.format(date);
 
-export const formatSessionDate = (value: string) => new Intl.DateTimeFormat(undefined, { weekday: 'short', month: 'short', day: 'numeric' }).format(dateFromKey(value));
+export const formatSessionDate = (value: string) => sessionDateFormatter.format(dateFromKey(value));
+
+export const formatWeekdayNarrow = (date: Date) => weekdayNarrowFormatter.format(date);
+
+export const formatWeekdayShort = (date: Date) => weekdayShortFormatter.format(date);
+
+export const formatWeekdayLong = (date: Date) => weekdayLongFormatter.format(date);
+
+export const formatMonthYear = (date: Date) => monthYearFormatter.format(date);
+
+export const formatMonthShortDay = (date: Date) => monthShortDayFormatter.format(date);
+
+export const formatYear = (date: Date) => yearFormatter.format(date);
+
+export const formatCurrentTime = (date = new Date()) => timeShortFormatter.format(date);
+
+export const formatHeaderDate = (date: Date) => headerDateFormatter.format(date);
 
 export const currentMinutes = () => {
   const now = new Date();