feat(day): single-column day view, wire into view switcher

Day view as a one-column slice of the week view: shared TimedBlock/
AllDaySpan layout, per-day swipe navigation, hoisted noon-centred scroll,
animated all-day strip, and a compact top bar showing the full date.

- DayUiState / DayViewModel / DayScreen under ui/day
- reuse layoutDay/layoutAllDay/coversDay from the week package
- add Day to IMPLEMENTED_VIEWS; CalendarHost routes it explicitly
- day_today_action strings (en/de)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/res/values-de/strings.xml

@@ -35,6 +35,9 @@
     <string name="week_today_action">Diese Woche</string>
     <string name="week_number_label">KW</string>
 
+    <!-- Tagesansicht (S3) -->
+    <string name="day_today_action">Heute</string>
+
     <!-- Geteilte Event-Strings -->
     <string name="event_untitled">(Ohne Titel)</string>