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/home/ViewHome.dart

@@ -1,5 +1,6 @@
 import '../../about/external/data/Commitment.dart';
 import '../../about/external/data/ExcuseCluster.dart';
+import '../../about/external/data/Group.dart';
 import '../../about/external/data/ReportCard.dart';
 import '../../about/external/data/RoutineChain.dart';
 import '../../about/external/data/pages/request/CommitmentsRequest.dart';
@@ -35,10 +36,10 @@ class ViewHome extends ParentViewModel {
 
     loadTodos();
 
-    loadWater();
-
     loadWeek();
 
+    loadGroups();
+
     loadPlan();
   }
 
@@ -61,12 +62,20 @@ class ViewHome extends ParentViewModel {
     }
   }
 
-  void loadWater() async {
-    connection.onWaterLoaded(await getDataManager().getWaterToday());
-  }
+  /// The groups rail. Quiet on failure: the rail just stays hidden.
+  void loadGroups() async {
+    try {
+      final response = await getDataManager().getMyGroups(HistoryRequest(
+        query: PageAndSort(
+          sort: Sort('desc', 'lastMessageAt'),
+          page: Pageable(0, 0, 20, 0),
+        ),
+      ));
 
-  void saveWater(int ml) async {
-    await getDataManager().setWaterToday(ml);
+      connection.onGroupsLoaded(getGroupList(response.data));
+    } catch (e) {
+      connection.onGroupsLoaded(<Group>[]);
+    }
   }
 
   void loadTodos() async {