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/mock/MockMemory.dart

@@ -19,9 +19,8 @@ class MockMemory extends InternalMemory {
       setMyDescription(
           MeDescription(id: "mock-user", name: "Alex", token: "mock-token"));
       setUserDetails(UserDetails(pic: "", name: "Alex"));
-      setWaterToday(1250);
       setTodos(<TodoItem>[
-        TodoItem(id: "t1", title: "Refill water bottle"),
+        TodoItem(id: "t1", title: "Pick up gym kit"),
         TodoItem(id: "t2", title: "Book physio for Thursday"),
         TodoItem(id: "t3", title: "Reply to Sam about Saturday", done: true),
       ]);