Add groups with shared goals, objectives and chat; drop the water tracker

Groups let people keep each other honest: create one or join with a
six-letter invite code. Each group has shared goals split into measurable
objectives (km, sessions, words) that every member logs toward, a
leaderboard of kept rate and streak with nudges, and a group chat where
check-ins and nudges appear as cards beside the talk. The open chat polls
for new messages.

Reached from a new Groups tab (Training moves into the day rail), a Home
"Your groups" rail that opens the chat, and a Group tile in the add sheet.
Mock mode seeds two groups, a joinable one and members who reply.

The water tracker is removed; it did not fit the app. Bottom sheets are
now square app-wide via the theme.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/lib/Grounded/about/external/data/Group.dart

@@ -0,0 +1,94 @@
+import '../../../utils/ObjectConvertors.dart';
+import 'GroupGoal.dart';
+import 'GroupMember.dart';
+
+/// People keeping each other honest: members, the goals they share, and the
+/// chat they talk in.
+class Group {
+  String id;
+
+  String name;
+
+  String description;
+
+  String colourHex;
+
+  /// Six characters a friend types to join.
+  String inviteCode;
+
+  List<GroupMember> members;
+
+  List<GroupGoal> goals;
+
+  /// Messages the signed-in user has not seen yet.
+  int unread;
+
+  String lastMessage;
+
+  String lastAuthor;
+
+  DateTime? lastMessageAt;
+
+  Group({
+    this.id = "",
+    this.name = "",
+    this.description = "",
+    this.colourHex = "",
+    this.inviteCode = "",
+    List<GroupMember>? members,
+    List<GroupGoal>? goals,
+    this.unread = 0,
+    this.lastMessage = "",
+    this.lastAuthor = "",
+    this.lastMessageAt,
+  })  : members = members ?? <GroupMember>[],
+        goals = goals ?? <GroupGoal>[];
+
+  /// Average progress across the shared goals.
+  double get progress {
+    if (goals.isEmpty) {
+      return 0;
+    }
+    double total = 0;
+    for (GroupGoal goal in goals) {
+      total = total + goal.progress;
+    }
+    return total / goals.length;
+  }
+
+  bool get iOwn => members.any((member) => member.me && member.owner);
+
+  factory Group.fromJson(Map<String, dynamic> json) {
+    return Group(
+      id: json['id'] ?? "",
+      name: json['name'] ?? "",
+      description: json['description'] ?? "",
+      colourHex: json['colourHex'] ?? "",
+      inviteCode: json['inviteCode'] ?? "",
+      members: getGroupMemberList(json['members']),
+      goals: getGroupGoalList(json['goals']),
+      unread: json['unread'] ?? 0,
+      lastMessage: json['lastMessage'] ?? "",
+      lastAuthor: json['lastAuthor'] ?? "",
+      lastMessageAt: json['lastMessageAt'] == null
+          ? null
+          : DateTime.tryParse(json['lastMessageAt']),
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    final Map<String, dynamic> data = <String, dynamic>{};
+    data['id'] = id;
+    data['name'] = name;
+    data['description'] = description;
+    data['colourHex'] = colourHex;
+    data['inviteCode'] = inviteCode;
+    data['members'] = members.map((item) => item.toJson()).toList();
+    data['goals'] = goals.map((item) => item.toJson()).toList();
+    data['unread'] = unread;
+    data['lastMessage'] = lastMessage;
+    data['lastAuthor'] = lastAuthor;
+    data['lastMessageAt'] = lastMessageAt?.toIso8601String();
+    return data;
+  }
+}