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/GroupGoal.dart

@@ -0,0 +1,67 @@
+import '../../../utils/ObjectConvertors.dart';
+import 'GroupObjective.dart';
+
+/// A goal the whole group shares, broken into objectives everyone logs
+/// against. Like a personal goal it carries no debt of its own.
+class GroupGoal {
+  String id;
+
+  String title;
+
+  String description;
+
+  DateTime? targetDate;
+
+  String colourHex;
+
+  List<GroupObjective> objectives;
+
+  GroupGoal({
+    this.id = "",
+    this.title = "",
+    this.description = "",
+    this.targetDate,
+    this.colourHex = "",
+    List<GroupObjective>? objectives,
+  }) : objectives = objectives ?? <GroupObjective>[];
+
+  /// The average of the objectives, so one runaway objective cannot carry
+  /// the rest.
+  double get progress {
+    if (objectives.isEmpty) {
+      return 0;
+    }
+    double total = 0;
+    for (GroupObjective objective in objectives) {
+      total = total + objective.progress;
+    }
+    return total / objectives.length;
+  }
+
+  int get completeObjectives =>
+      objectives.where((objective) => objective.complete).length;
+
+  factory GroupGoal.fromJson(Map<String, dynamic> json) {
+    return GroupGoal(
+      id: json['id'] ?? "",
+      title: json['title'] ?? "",
+      description: json['description'] ?? "",
+      targetDate: json['targetDate'] == null || json['targetDate'] == ""
+          ? null
+          : DateTime.tryParse(json['targetDate']),
+      colourHex: json['colourHex'] ?? "",
+      objectives: getGroupObjectiveList(json['objectives']),
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    final Map<String, dynamic> data = <String, dynamic>{};
+    data['id'] = id;
+    data['title'] = title;
+    data['description'] = description;
+    data['targetDate'] = targetDate?.toIso8601String();
+    data['colourHex'] = colourHex;
+    data['objectives'] = objectives.map((item) => item.toJson()).toList();
+    return data;
+  }
+}