fix(calendar): Monday-first Swedish week, today highlight, default to current month

- Convert week calculation to Monday-first (startOffset = (firstDay.getDay() + 6) % 7)
- Update weekday headers to Swedish labels: [Mån, Tis, Ons, Tor, Fre, Lör, Sön]
- Change default currentMonth from June 2026 to current date (new Date())
- Add isToday prop to CalendarDay component
- Add --today modifier to highlight current date with blue circle background
- Pass computed isToday boolean to CalendarDay when date equals today's date string
- Renders correctly for Sunday-starting months (e.g., March 2026)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/components/common/CalendarDay.tsx

@@ -2,9 +2,10 @@ import { ReactNode } from 'react'
 
 export interface CalendarDayProps {
   date: string
-  events?: { type: string; color: string; title?: string }[]
+  events?: { type: string; color: string; title?: string }[] | undefined
-  isCurrentMonth?: boolean
+  isCurrentMonth?: boolean | undefined
-  onClick?: () => void
+  onClick?: (() => void) | undefined
+  isToday?: boolean | undefined
 }
 
 export function CalendarDay({
@@ -12,6 +13,7 @@ export function CalendarDay({
   events = [],
   isCurrentMonth = true,
   onClick,
+  isToday = false,
 }: CalendarDayProps): ReactNode {
   const dayNum = new Date(date + 'T00:00:00Z').getUTCDate()
 
@@ -22,7 +24,7 @@ export function CalendarDay({
       role="button"
       tabIndex={0}
       aria-label={`${date}${events.length > 0 ? ` with ${events.length} event(s)` : ''}`}
-      className={`calendar-day ${!isCurrentMonth ? 'calendar-day--disabled' : ''} ${events.length > 0 ? 'calendar-day--has-events' : ''}`}
+      className={`calendar-day ${!isCurrentMonth ? 'calendar-day--disabled' : ''} ${events.length > 0 ? 'calendar-day--has-events' : ''} ${isToday ? 'calendar-day--today' : ''}`}
       onKeyDown={(e) => {
         if (e.key === 'Enter' || e.key === ' ') {
           onClick?.()