fix: unify date limits, fix ISO week calculation, refactor home screen plan button

- Fix _isoWeek: correct Sunday shift (-3 instead of +4), use floor+1 formula,
  match jan1 timezone to input — planned meals now appear correctly for UTC+ users
- Add kPlanningHorizonDays=28 / kMenuPastWeeks=8 constants; apply to home date
  strip, plan picker (strip + calendar), and menu screen prev/next navigation
- Menu screen week nav: disable arrows at min/max limits using compareTo
- Home screen: replace _GenerateActionCard/_WeekPlannedChip conditional with
  always-visible _FutureDayPlanButton(dateString); show _DayPlannedChip only
  when the specific day has planned meals; remove standalone _PlanMenuButton
- _FutureDayPlanButton uses selected date as defaultStart instead of lastPlanned+1
- Rename weekPlannedLabel -> dayPlannedLabel across all 12 locales

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/features/menu/menu_provider.dart

@@ -16,22 +16,27 @@ final menuServiceProvider = Provider<MenuService>((ref) {
 
 /// The ISO week string for the currently displayed week, e.g. "2026-W08".
 final currentWeekProvider = StateProvider<String>((ref) {
-  final now = DateTime.now().toUtc();
+  final now = DateTime.now();
   final (y, w) = _isoWeek(now);
   return '$y-W${w.toString().padLeft(2, '0')}';
 });
 
 (int year, int week) _isoWeek(DateTime dt) {
-  // Shift to Thursday to get ISO week year.
-  final thu = dt.add(Duration(days: 4 - (dt.weekday == 7 ? 0 : dt.weekday)));
-  final jan1 = DateTime.utc(thu.year, 1, 1);
-  final week = ((thu.difference(jan1).inDays) / 7).ceil();
+  // Shift to Thursday of the same ISO week.
+  // Monday=1…Saturday=6 → add (4 - weekday) days; Sunday=7 → subtract 3 days.
+  final int shift = dt.weekday == 7 ? -3 : 4 - dt.weekday;
+  final thu = dt.add(Duration(days: shift));
+  // Use the same timezone as the input to avoid offset drift on the difference.
+  final jan1 = dt.isUtc
+      ? DateTime.utc(thu.year, 1, 1)
+      : DateTime(thu.year, 1, 1);
+  final week = (thu.difference(jan1).inDays ~/ 7) + 1;
   return (thu.year, week);
 }
 
 /// Returns the ISO 8601 week string for [date], e.g. "2026-W12".
 String isoWeekString(DateTime date) {
-  final (year, week) = _isoWeek(date.toUtc());
+  final (year, week) = _isoWeek(date);
   return '$year-W${week.toString().padLeft(2, '0')}';
 }