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/designs/MemberAvatar.dart

@@ -0,0 +1,74 @@
+import 'package:flutter/material.dart';
+
+import '../about/internal/application/TextType.dart';
+import '../utils/Colors.dart';
+import '../utils/CommonUtils.dart';
+import 'scene/SceneArt.dart';
+import 'text/Text.dart';
+
+/// A member's square portrait: a landscape picked from their name with the
+/// initial on it, so everyone is recognisable without a photo.
+Widget memberAvatar(String name, double size, {Color? border}) {
+  const List<List<Color>> palettes = <List<Color>>[
+    sceneWorkout,
+    sceneMeditation,
+    sceneMustDo,
+  ];
+  final int seed = sceneSeed(name.isEmpty ? "?" : name);
+  final String initial =
+      name.isEmpty ? "?" : name.substring(0, 1).toUpperCase();
+
+  return Container(
+    width: size,
+    height: size,
+    decoration:
+        BoxDecoration(border: Border.all(color: border ?? colorWhite, width: 2)),
+    child: Stack(
+      fit: StackFit.expand,
+      children: [
+        SceneArt(palette: palettes[seed % palettes.length], seed: seed),
+        Center(
+          child: text(initial, size * 0.42, TextType.Medium, color: colorWhite),
+        ),
+      ],
+    ),
+  );
+}
+
+/// Several portraits overlapping, with "+N" once there are more than fit.
+Widget avatarStack(List<String> names, double size,
+    {int max = 4, Color? border}) {
+  final List<String> shown = names.take(max).toList();
+  final int extra = names.length - shown.length;
+  final double step = size * 0.68;
+  final int slots = shown.length + (extra > 0 ? 1 : 0);
+
+  return SizedBox(
+    width: slots == 0 ? 0 : size + step * (slots - 1),
+    height: size,
+    child: Stack(
+      children: [
+        for (int index = 0; index < shown.length; index++)
+          Positioned(
+            left: step * index,
+            child: memberAvatar(shown[index], size, border: border),
+          ),
+        if (extra > 0)
+          Positioned(
+            left: step * shown.length,
+            child: Container(
+              width: size,
+              height: size,
+              alignment: Alignment.center,
+              decoration: BoxDecoration(
+                color: colorNight,
+                border: Border.all(color: border ?? colorWhite, width: 2),
+              ),
+              child: text("+$extra", size * 0.34, TextType.Medium,
+                  color: colorWhite),
+            ),
+          ),
+      ],
+    ),
+  );
+}