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/ViewGoalDetail.dart

@@ -1,5 +1,6 @@
 import '../../about/external/data/Commitment.dart';
 import '../../about/external/data/Goal.dart';
+import '../../about/external/initial/CompletionRequest.dart';
 import '../../about/external/initial/IdRequest.dart';
 import '../../utils/ObjectConvertors.dart';
 import '../parent/ParentViewModel.dart';
@@ -27,6 +28,23 @@ class ViewGoalDetail extends ParentViewModel {
     }
   }
 
+  /// Completes a task straight from the goal list.
+  void tick(Commitment task) async {
+    if (!await hasNetwork(() => tick(task))) return;
+
+    try {
+      await getDataManager().completeCommitmentEntry(CompletionRequest(
+        commitmentId: task.id ?? "",
+        proofType: task.proofType.name,
+      ));
+
+      connection.onTaskTicked(task, task.wouldBeLate);
+    } catch (e) {
+      connection.onTickFailed(task);
+      handleError(e, () => tick(task), () => dismissError(), "Retry");
+    }
+  }
+
   /// Stashes the task as the active one before the runner opens, so the
   /// ongoing notification and any relaunch land back on the right thing.
   void startTask(Commitment task) async {