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/comms/CommsDirections.dart

@@ -172,3 +172,24 @@ String myPartners = 'Partner/Mine';
 String invitePartner = 'Partner/Invite';
 
 String partnerPolicy = 'Partner/Policy';
+
+// ── Groups ────────────────────────────────────────────────────────────────────
+String myGroups = 'Group/Mine';
+
+String oneGroup = 'Group/One';
+
+String saveGroup = 'Group/Save';
+
+String joinGroup = 'Group/Join';
+
+String leaveGroup = 'Group/Leave';
+
+String saveGroupGoal = 'Group/Goal/Save';
+
+String logObjective = 'Group/Objective/Log';
+
+String groupMessages = 'Group/Messages';
+
+String sendGroupMessageTo = 'Group/Message/Send';
+
+String nudgeMember = 'Group/Nudge';