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/GroupMessage.dart

@@ -0,0 +1,66 @@
+/// One line in a group chat. Besides plain text, the app posts check-ins
+/// (progress logged on an objective), nudges and system notes into the same
+/// thread so the conversation and the work sit together.
+class GroupMessage {
+  String id;
+
+  String groupId;
+
+  String authorId;
+
+  String authorName;
+
+  String body;
+
+  /// "Text", "CheckIn", "Nudge" or "System".
+  String kind;
+
+  DateTime? sentAt;
+
+  /// True when the signed-in user wrote it.
+  bool mine;
+
+  GroupMessage({
+    this.id = "",
+    this.groupId = "",
+    this.authorId = "",
+    this.authorName = "",
+    this.body = "",
+    this.kind = "Text",
+    this.sentAt,
+    this.mine = false,
+  });
+
+  bool get isCheckIn => kind == "CheckIn";
+
+  bool get isNudge => kind == "Nudge";
+
+  bool get isSystem => kind == "System";
+
+  factory GroupMessage.fromJson(Map<String, dynamic> json) {
+    return GroupMessage(
+      id: json['id'] ?? "",
+      groupId: json['groupId'] ?? "",
+      authorId: json['authorId'] ?? "",
+      authorName: json['authorName'] ?? "",
+      body: json['body'] ?? "",
+      kind: json['kind'] ?? "Text",
+      sentAt:
+          json['sentAt'] == null ? null : DateTime.tryParse(json['sentAt']),
+      mine: json['mine'] ?? false,
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    final Map<String, dynamic> data = <String, dynamic>{};
+    data['id'] = id;
+    data['groupId'] = groupId;
+    data['authorId'] = authorId;
+    data['authorName'] = authorName;
+    data['body'] = body;
+    data['kind'] = kind;
+    data['sentAt'] = sentAt?.toIso8601String();
+    data['mine'] = mine;
+    return data;
+  }
+}