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/utils/ObjectConvertors.dart

@@ -4,6 +4,11 @@ import '../about/external/data/DebtEntry.dart';
 import '../about/external/data/ExcuseCluster.dart';
 import '../about/external/data/ExercisePrescription.dart';
 import '../about/external/data/Goal.dart';
+import '../about/external/data/Group.dart';
+import '../about/external/data/GroupGoal.dart';
+import '../about/external/data/GroupMember.dart';
+import '../about/external/data/GroupMessage.dart';
+import '../about/external/data/GroupObjective.dart';
 import '../about/external/data/Habit.dart';
 import '../about/external/data/Program.dart';
 import '../about/external/data/RoutineChain.dart';
@@ -122,3 +127,38 @@ List<String> getStringList(dynamic data) {
   }
   return (data as List).map((item) => item.toString()).toList();
 }
+
+List<Group> getGroupList(dynamic data) {
+  if (data == null) {
+    return <Group>[];
+  }
+  return (data as List).map((item) => Group.fromJson(item)).toList();
+}
+
+List<GroupMember> getGroupMemberList(dynamic data) {
+  if (data == null) {
+    return <GroupMember>[];
+  }
+  return (data as List).map((item) => GroupMember.fromJson(item)).toList();
+}
+
+List<GroupGoal> getGroupGoalList(dynamic data) {
+  if (data == null) {
+    return <GroupGoal>[];
+  }
+  return (data as List).map((item) => GroupGoal.fromJson(item)).toList();
+}
+
+List<GroupObjective> getGroupObjectiveList(dynamic data) {
+  if (data == null) {
+    return <GroupObjective>[];
+  }
+  return (data as List).map((item) => GroupObjective.fromJson(item)).toList();
+}
+
+List<GroupMessage> getGroupMessageList(dynamic data) {
+  if (data == null) {
+    return <GroupMessage>[];
+  }
+  return (data as List).map((item) => GroupMessage.fromJson(item)).toList();
+}