Extract the shared per-calendar event-collection logic out of
buildMonthView into collectCalendarEvents, keyed by day so it can feed
either the existing 42-cell month grid or a new 7-cell week grid.
Add templates.WeekDay/WeekViewData, a WeekView templ, and a
viewSwitcher component shown in both views' headers to switch between
Month and Week. Add buildWeekView, handleCalendarWeek, parseWeekStart,
and mondayOf in internal/web/calendar.go, and register the new
/calendar/week route.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>