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/l10n/app_localizations.dart

@@ -826,11 +826,11 @@ abstract class AppLocalizations {
   /// **'Generating menu...'**
   String get generatingMenu;
 
-  /// No description provided for @weekPlannedLabel.
+  /// No description provided for @dayPlannedLabel.
   ///
   /// In en, this message translates to:
-  /// **'Week planned'**
-  String get weekPlannedLabel;
+  /// **'Day planned'**
+  String get dayPlannedLabel;
 
   /// No description provided for @planMenuButton.
   ///