Add tick boxes to task rows

Tasks now carry a tick box rather than being read-only rows. Ticking strikes
the title through and leaves the item in place, so a day still reads as a
record of what was asked rather than only what is left.

New in designs/Checkbox.dart:

- TickBox   the square box itself, animating between empty and filled
- TickRow   box + struck-through title + caller-supplied detail and trailing
- addTaskAffordance  the centred "Add new task" control beneath a day's list

Applied to the home plan, the overdue queue and the goal task list. Each
screen holds a set of optimistically ticked ids so the tick lands immediately
and rolls back if the call fails; the set is reconciled whenever fresh data
arrives.

A tick completes the item outright, whatever its proof requirement. The proof
flow is still reachable from the Complete button in the overdue queue, so
tasks that need an artefact have a route that asks for one — but the tick
itself no longer enforces it.

Late completes stay visually distinct: the row keeps a Late pill and the
warning accent, and the user is told the window had already closed.

flutter analyze lib/ reports no errors.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mfu2gQLSFN21YRBcU2NrTt

frontend/lib/Grounded/see/goal/ConnectGoalDetail.dart

@@ -6,4 +6,10 @@ abstract class ConnectGoalDetail {
 
   /// The task is ready to run — hand off to the full-screen runner.
   void onTaskReady(Commitment task);
+
+  /// A tick landed. [late] is true when the window had already closed.
+  void onTaskTicked(Commitment task, bool late);
+
+  /// The tick failed; the row goes back to unchecked.
+  void onTickFailed(Commitment task);
 }