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/see/group/ViewGroupGoalEditor.dart

@@ -0,0 +1,25 @@
+import '../../about/external/initial/GroupGoalRequest.dart';
+import '../parent/ParentViewModel.dart';
+import 'ConnectGroupGoalEditor.dart';
+
+class ViewGroupGoalEditor extends ParentViewModel {
+  ConnectGroupGoalEditor connection;
+
+  ViewGroupGoalEditor(super.context, this.connection);
+
+  void save(GroupGoalRequest request) async {
+    if (!await hasNetwork(() => save(request))) return;
+
+    showLoading("Saving the shared goal");
+
+    try {
+      await getDataManager().saveGroupGoalEntry(request);
+
+      closeLoading();
+
+      connection.onGoalSaved();
+    } catch (e) {
+      handleError(e, () => save(request), () => dismissError(), "Retry");
+    }
+  }
+}