feat(agenda): add Agenda view — upcoming events grouped by day

The fourth top-level view, alongside Month/Week/Day. A forward-looking
LazyColumn of upcoming events grouped under sticky day headers, reusing
the v2.3 grouped-list language (GroupedRow cards, color-rail leading).

- AgendaViewModel loads a 60-day forward window from the anchor day
  (today by default; goToToday/goToDate drive the FAB + drawer jump),
  groups instances by local day (ongoing/multi-day clamped to the
  anchor), sorts all-day-first then by start.
- AgendaScreen: same drawer + scaffold + view-switcher + FAB shell as
  Day; sticky "Today · …"/"Tomorrow · …" headers, event rows with
  time·location, plus empty/failure/loading states.
- Wired into CalendarView (ViewAgenda icon), IMPLEMENTED_VIEWS, and
  CalendarHost; strings added (EN + DE).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/ui/CalendarHost.kt

@@ -16,6 +16,7 @@ import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import de.jeanlucmakiola.calendula.domain.EventInstance
+import de.jeanlucmakiola.calendula.ui.agenda.AgendaScreen
 import de.jeanlucmakiola.calendula.ui.calendars.CalendarsScreen
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
 import de.jeanlucmakiola.calendula.ui.common.rememberCalendarSlideSpec
@@ -141,6 +142,13 @@ fun CalendarHost(
                 onOpenSettings = onOpenSettings,
                 onCreateEvent = onCreateEvent,
             )
+            CalendarView.Agenda -> AgendaScreen(
+                selectedView = view,
+                onSelectView = onSelectView,
+                onEventClick = onEventClick,
+                onOpenSettings = onOpenSettings,
+                onCreateEvent = onCreateEvent,
+            )
         }
 
         // Prefer the live key; fall back to the held one only while sliding out.