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/GroupObjective.dart

@@ -0,0 +1,58 @@
+/// One measurable part of a shared goal — "Run 100 km", "20 sessions" — that
+/// every member logs toward.
+class GroupObjective {
+  String id;
+
+  String title;
+
+  /// km, sessions, pages, hours…
+  String unit;
+
+  double target;
+
+  /// The group's total so far.
+  double current;
+
+  /// What the signed-in user has put in.
+  double mine;
+
+  GroupObjective({
+    this.id = "",
+    this.title = "",
+    this.unit = "",
+    this.target = 0,
+    this.current = 0,
+    this.mine = 0,
+  });
+
+  double get progress {
+    if (target <= 0) {
+      return 0;
+    }
+    return (current / target).clamp(0.0, 1.0);
+  }
+
+  bool get complete => target > 0 && current >= target;
+
+  factory GroupObjective.fromJson(Map<String, dynamic> json) {
+    return GroupObjective(
+      id: json['id'] ?? "",
+      title: json['title'] ?? "",
+      unit: json['unit'] ?? "",
+      target: (json['target'] ?? 0).toDouble(),
+      current: (json['current'] ?? 0).toDouble(),
+      mine: (json['mine'] ?? 0).toDouble(),
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    final Map<String, dynamic> data = <String, dynamic>{};
+    data['id'] = id;
+    data['title'] = title;
+    data['unit'] = unit;
+    data['target'] = target;
+    data['current'] = current;
+    data['mine'] = mine;
+    return data;
+  }
+}