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/ConnectHome.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/internal/application/Standing.dart';
@@ -20,8 +21,8 @@ abstract class ConnectHome {
   /// view. Null when it could not be loaded.
   void onWeekLoaded(ReportCard? week);
 
-  /// Water drunk today, in ml.
-  void onWaterLoaded(int ml);
+  /// Your groups, for the rail on Home.
+  void onGroupsLoaded(List<Group> groups);
 
   /// The quick to-do list, read from the device.
   void onTodosLoaded(List<TodoItem> todos);