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/design.md

@@ -88,5 +88,24 @@ minutes held against planned, a ring with + that opens the "Add to today"
 sheet, the plan as thumbnails, the report-card letter as the Score, and one
 bar per commitment class. Weekly swaps in the week's average, a bar per day
 and the class completion rates. Below the block, on the light ambient
-surface: the water tracker (drops of 250 ml, kept on the device), then the
-rest of the day. Bars on the dark block use the `colorBar*` tokens.
+surface: the rest of the day. Bars on the dark block use the `colorBar*` tokens.
+
+## Groups
+
+`see/group/`: people keeping each other honest. A group has members, shared
+goals and a chat.
+
+- **Shared goals** are split into **objectives** with a number and a unit
+  (200 km, 40 runs, 30,000 words). Every member logs toward them; the card
+  shows the group total, your share and a night **Log** button. Like
+  personal goals, they carry no debt.
+- **Leaderboard** ranks members by kept rate over the last 7 days, with
+  streak, last check-in (red "quiet" after 24h) and a nudge button.
+- **Chat** keeps the work and the talk together: plain messages are square
+  bubbles (yours in night, others white with the author's colour), while
+  check-ins (green bar) and nudges (amber bar) are cards in the thread.
+  The open chat polls every 4 seconds.
+- Members are drawn by `memberAvatar` / `avatarStack`
+  (`designs/MemberAvatar.dart`): square painted portraits with the initial.
+- Groups are reached from the nav (Groups tab), the Home "Your groups" rail
+  (tap opens the chat) and the add sheet.