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/utils/CommonUtils.dart

@@ -404,3 +404,20 @@ int sceneSeed(String key) {
   }
   return hash;
 }
+
+/// An amount the way a person reads it: whole numbers with thousands
+/// separators, anything else to one decimal place.
+String formatAmount(double value) {
+  if (value == value.roundToDouble()) {
+    final String digits = value.round().abs().toString();
+    final StringBuffer out = StringBuffer(value < 0 ? "-" : "");
+    for (int index = 0; index < digits.length; index++) {
+      if (index > 0 && (digits.length - index) % 3 == 0) {
+        out.write(",");
+      }
+      out.write(digits[index]);
+    }
+    return out.toString();
+  }
+  return value.toStringAsFixed(1);
+}