refactor(week): polish timeline — rounded viewport, scroll persistence, week badge

- Rounded, permanently-soft day-column scroll viewport via two viewports
  sharing one scroll state (gutter + columns stay aligned); plain
  rectangular column cards inside
- Vertical scroll position now persists across week swipes; noon-centring
  only runs on first entry into the week view (from month/day)
- All-day strip height is hoisted + animated, shared by both swipe pages,
  so it slides along and resizes smoothly instead of jumping
- Multi-line event time label so the end time isn't clipped in narrow
  columns; hour labels centred in the gutter
- Calendar-week (ISO) badge in the header gutter, aligned with the date
  numbers; dropped the redundant "All-day" gutter label
- Small breathing room between the top section and the timeline

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

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

@@ -32,8 +32,8 @@
     <string name="month_a11y_today_prefix">Heute</string>
 
     <!-- Wochenansicht (S2) -->
-    <string name="week_all_day">Ganztägig</string>
     <string name="week_today_action">Diese Woche</string>
+    <string name="week_number_label">KW</string>
 
     <!-- Geteilte Event-Strings -->
     <string name="event_untitled">(Ohne Titel)</string>