feat: add sorting by category for checklist

lib/messages.i18n.dart

@@ -661,6 +661,11 @@ class SortChecklistsMessages {
   /// ```
   String get nameZA => """Name Z–A""";
 
+  /// ```dart
+  /// "By category"
+  /// ```
+  String get category => """By category""";
+
   /// ```dart
   /// "Custom"
   /// ```
@@ -1231,6 +1236,7 @@ Please complete login in your browser.""",
   """checklists.sort.oldestFirst""": """Oldest first""",
   """checklists.sort.nameAZ""": """Name A–Z""",
   """checklists.sort.nameZA""": """Name Z–A""",
+  """checklists.sort.category""": """By category""",
   """checklists.sort.custom""": """Custom""",
   """notesWall.noNotes""": """No notes yet.""",
   """notesWall.failedToLoad""": """Failed to load notes.""",

lib/messages.i18n.yaml

@@ -120,6 +120,7 @@ checklists:
     oldestFirst: Oldest first
     nameAZ: "Name A\u2013Z"
     nameZA: "Name Z\u2013A"
+    category: By category
     custom: Custom
 
 notesWall:

lib/widgets/checklist_sort_button.dart

@@ -17,6 +17,7 @@ class ChecklistSortButton extends StatelessWidget {
     'oldest',
     'name_asc',
     'name_desc',
+    'category',
     'custom',
   ];
 
@@ -25,6 +26,7 @@ class ChecklistSortButton extends StatelessWidget {
     'oldest' => m.checklists.sort.oldestFirst,
     'name_asc' => m.checklists.sort.nameAZ,
     'name_desc' => m.checklists.sort.nameZA,
+    'category' => m.checklists.sort.category,
     'custom' => m.checklists.sort.custom,
     _ => key,
   };