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/GroupMember.dart

@@ -0,0 +1,64 @@
+/// Someone in a group, with the numbers the leaderboard ranks on.
+class GroupMember {
+  String id;
+
+  String name;
+
+  /// "Owner" or "Member".
+  String role;
+
+  /// Share of their own commitments kept over the last seven days, 0..1.
+  double keptRate;
+
+  /// Kept days in a row.
+  int streak;
+
+  /// Their standing name — Good, Warned, Grounded, Lockdown.
+  String standing;
+
+  DateTime? lastCheckIn;
+
+  /// True for the signed-in user.
+  bool me;
+
+  GroupMember({
+    this.id = "",
+    this.name = "",
+    this.role = "Member",
+    this.keptRate = 0,
+    this.streak = 0,
+    this.standing = "Good",
+    this.lastCheckIn,
+    this.me = false,
+  });
+
+  bool get owner => role == "Owner";
+
+  factory GroupMember.fromJson(Map<String, dynamic> json) {
+    return GroupMember(
+      id: json['id'] ?? "",
+      name: json['name'] ?? "",
+      role: json['role'] ?? "Member",
+      keptRate: (json['keptRate'] ?? 0).toDouble(),
+      streak: json['streak'] ?? 0,
+      standing: json['standing'] ?? "Good",
+      lastCheckIn: json['lastCheckIn'] == null
+          ? null
+          : DateTime.tryParse(json['lastCheckIn']),
+      me: json['me'] ?? false,
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    final Map<String, dynamic> data = <String, dynamic>{};
+    data['id'] = id;
+    data['name'] = name;
+    data['role'] = role;
+    data['keptRate'] = keptRate;
+    data['streak'] = streak;
+    data['standing'] = standing;
+    data['lastCheckIn'] = lastCheckIn?.toIso8601String();
+    data['me'] = me;
+    return data;
+  }
+}