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_de.arb

@@ -135,7 +135,7 @@
   "generateWeekLabel": "Woche planen",
   "generateWeekSubtitle": "KI erstellt einen Menüplan mit Frühstück, Mittagessen und Abendessen für die ganze Woche",
   "generatingMenu": "Menü wird erstellt...",
-  "weekPlannedLabel": "Woche geplant",
+  "dayPlannedLabel": "Tag geplant",
 
   "planMenuButton": "Mahlzeiten planen",
   "planMenuTitle": "Was planen?",