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')}';
 }
 

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);
+                },
         ),
       ],
     );

client/lib/features/menu/plan_date_picker_sheet.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:food_ai/l10n/app_localizations.dart';
 import 'package:intl/intl.dart';
 
+import '../../shared/constants/date_limits.dart';
 import '../../shared/models/meal_type.dart';
 import '../profile/profile_provider.dart';
 import 'menu_provider.dart';
@@ -279,8 +280,8 @@ class _DateStripSelector extends StatefulWidget {
 
 class _DateStripSelectorState extends State<_DateStripSelector> {
   late final ScrollController _scrollController;
-  // Show 30 upcoming days (today excluded, starts tomorrow).
-  static const _futureDays = 30;
+  // Show upcoming days up to the planning horizon (today excluded, starts tomorrow).
+  static const _futureDays = kPlanningHorizonDays;
   static const _itemWidth = 64.0;
 
   DateTime get _tomorrow =>
@@ -407,12 +408,22 @@ class _CalendarRangePickerState extends State<_CalendarRangePicker> {
   }
 
   void _nextMonth() {
+    final horizon = DateTime.now().add(const Duration(days: kPlanningHorizonDays));
+    final limitMonth = DateTime(horizon.year, horizon.month);
+    if (!DateTime(_displayMonth.year, _displayMonth.month).isBefore(limitMonth)) return;
     setState(() {
       _displayMonth =
           DateTime(_displayMonth.year, _displayMonth.month + 1);
     });
   }
 
+  bool _isBeyondHorizon(DateTime date) {
+    final today = DateTime.now();
+    final horizon = DateTime(today.year, today.month, today.day)
+        .add(const Duration(days: kPlanningHorizonDays));
+    return date.isAfter(horizon);
+  }
+
   bool _isInRange(DateTime date) {
     final dayOnly = DateTime(date.year, date.month, date.day);
     final start =
@@ -510,11 +521,13 @@ class _CalendarRangePickerState extends State<_CalendarRangePicker> {
             final isStart = _isRangeStart(date);
             final isEnd = _isRangeEnd(date);
             final isPast = _isPast(date);
+            final isBeyond = _isBeyondHorizon(date);
+            final isDisabled = isPast || isBeyond;
 
             Color bgColor = Colors.transparent;
             Color textColor = theme.colorScheme.onSurface;
 
-            if (isPast) {
+            if (isDisabled) {
               // ignore: deprecated_member_use
               textColor = theme.colorScheme.onSurface.withOpacity(0.3);
             } else if (isStart || isEnd) {
@@ -526,7 +539,7 @@ class _CalendarRangePickerState extends State<_CalendarRangePicker> {
             }
 
             return GestureDetector(
-              onTap: isPast ? null : () => widget.onDayTapped(date),
+              onTap: isDisabled ? null : () => widget.onDayTapped(date),
               child: Container(
                 decoration: BoxDecoration(
                   color: bgColor,