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_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import '../../shared/constants/date_limits.dart';
 import '../../shared/models/menu.dart';
 import 'menu_provider.dart';
 
@@ -95,31 +96,48 @@ class _WeekNavBar extends StatelessWidget implements PreferredSizeWidget {
   }
 
   (int, int) _isoWeekOf(DateTime dt) {
-    final thu = dt.add(Duration(days: 4 - (dt.weekday == 7 ? 0 : dt.weekday)));
-    final jan1 = DateTime.utc(thu.year, 1, 1);
-    final w = ((thu.difference(jan1).inDays) / 7).ceil();
+    final int shift = dt.weekday == 7 ? -3 : 4 - dt.weekday;
+    final thu = dt.add(Duration(days: shift));
+    final jan1 = dt.isUtc
+        ? DateTime.utc(thu.year, 1, 1)
+        : DateTime(thu.year, 1, 1);
+    final w = (thu.difference(jan1).inDays ~/ 7) + 1;
     return (thu.year, w);
   }
 
+  String _currentWeekString(DateTime date) {
+    final (y, w) = _isoWeekOf(date);
+    return '$y-W${w.toString().padLeft(2, '0')}';
+  }
+
   @override
   Widget build(BuildContext context) {
+    final now = DateTime.now();
+    final maxWeek = _currentWeekString(
+        now.add(const Duration(days: kPlanningHorizonDays)));
+    final minWeek = _currentWeekString(
+        now.subtract(Duration(days: kMenuPastWeeks * 7)));
     return Row(
       mainAxisAlignment: MainAxisAlignment.center,
       children: [
         IconButton(
           icon: const Icon(Icons.chevron_left),
-          onPressed: () {
-            ref.read(currentWeekProvider.notifier).state =
-                _offsetWeek(week, -1);
-          },
+          onPressed: week.compareTo(minWeek) <= 0
+              ? null
+              : () {
+                  ref.read(currentWeekProvider.notifier).state =
+                      _offsetWeek(week, -1);
+                },
         ),
         Text(_weekLabel(week), style: Theme.of(context).textTheme.bodyMedium),
         IconButton(
           icon: const Icon(Icons.chevron_right),
-          onPressed: () {
-            ref.read(currentWeekProvider.notifier).state =
-                _offsetWeek(week, 1);
-          },
+          onPressed: week.compareTo(maxWeek) >= 0
+              ? null
+              : () {
+                  ref.read(currentWeekProvider.notifier).state =
+                      _offsetWeek(week, 1);
+                },
         ),
       ],
     );