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/res/values-de/strings.xml

@@ -196,11 +196,19 @@
     <string name="view_month">Monat</string>
     <string name="view_week">Woche</string>
     <string name="view_day">Tag</string>
+    <string name="view_agenda">Agenda</string>
     <string name="view_section">Ansicht</string>
 
     <!-- Zu Datum springen (Navigationsleiste) -->
     <string name="drawer_jump_to_date">Zu Datum springen</string>
 
+    <!-- Agenda-Ansicht -->
+    <string name="agenda_today_action">Heute</string>
+    <string name="agenda_header_today">Heute</string>
+    <string name="agenda_header_tomorrow">Morgen</string>
+    <string name="agenda_empty_title">Nichts geplant</string>
+    <string name="agenda_empty_subtitle">Anstehende Termine erscheinen hier.</string>
+
     <!-- Kalender-Filter (M3) -->
     <string name="filter_title">Kalender</string>