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/ConnectGroupChat.dart

@@ -0,0 +1,10 @@
+import '../../about/external/data/GroupMessage.dart';
+
+abstract class ConnectGroupChat {
+  void onMessagesLoaded(List<GroupMessage> messages);
+
+  void onMessageSent(GroupMessage message);
+
+  /// The send failed; the text goes back in the box so nothing is lost.
+  void onSendFailed(String body);
+}