feat: add day detail screen with swipe-to-delete and undo

Adds DayDetailScreen showing all entries for a selected day with
swipe-to-delete and an undo snackbar. CalendarScreen gains a tappable
day header (Today / Yesterday / Month Day) that pushes the detail screen.

lib/features/calendar/calendar_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../../core/providers/mood_provider.dart';
+import 'day_detail_screen.dart';
 import 'widgets/calendar_heatmap.dart';
 import 'widgets/day_entry_card.dart';
 
@@ -36,6 +37,15 @@ class _CalendarScreenState extends ConsumerState<CalendarScreen> {
     }
   }
 
+  static String _dayLabel(DateTime day) {
+    final now = DateTime.now();
+    final today = DateTime(now.year, now.month, now.day);
+    final yesterday = today.subtract(const Duration(days: 1));
+    if (day == today) return 'Today';
+    if (day == yesterday) return 'Yesterday';
+    return '${_months[day.month - 1]} ${day.day}';
+  }
+
   bool get _canGoForward {
     final now = DateTime.now();
     return _month.year < now.year ||
@@ -70,6 +80,28 @@ class _CalendarScreenState extends ConsumerState<CalendarScreen> {
             },
           ),
           const Divider(height: 1),
+          InkWell(
+            onTap: () => Navigator.of(context).push(
+              MaterialPageRoute(
+                builder: (_) => DayDetailScreen(day: selectedDay),
+              ),
+            ),
+            child: Padding(
+              padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 10),
+              child: Row(
+                children: [
+                  Text(
+                    _dayLabel(selectedDay),
+                    style: Theme.of(context).textTheme.titleSmall,
+                  ),
+                  const Spacer(),
+                  Icon(Icons.chevron_right,
+                      color: Theme.of(context).colorScheme.outline),
+                ],
+              ),
+            ),
+          ),
+          const Divider(height: 1),
           Expanded(
             child: entriesAsync.when(
               data: (entries) => entries.isEmpty