Square every corner, enlarge the main surfaces and add a to-do list

Rounded rectangles and round controls are now square across the app; only
status dots, glows, the sun, rings and the breathing circle stay round.
Cards, rows, thumbnails, featured cards, category tiles, the standing card
and the nav are larger. Home gets a quick To-do list kept on the device
(TodoItem in InternalMemory), seeded with sample items in mock mode.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KhQh614DMeYoQEH9SoMRUw

frontend/lib/Grounded/designs/Checkbox.dart

@@ -48,7 +48,7 @@ class TickBox extends StatelessWidget {
           height: size,
           decoration: BoxDecoration(
             color: checked ? tone : Colors.transparent,
-            borderRadius: BorderRadius.circular(7),
+            borderRadius: BorderRadius.zero,
             border: Border.all(
               color: checked ? tone : colorGrey.withValues(alpha: 0.55),
               width: 1.6,
@@ -202,7 +202,7 @@ Widget addTaskAffordance(
             alignment: Alignment.center,
             decoration: BoxDecoration(
               color: tone.withValues(alpha: 0.12),
-              borderRadius: BorderRadius.circular(14),
+              borderRadius: BorderRadius.zero,
               border: Border.all(color: tone.withValues(alpha: 0.25), width: 1),
             ),
             child: Icon(Icons.add_rounded, size: 22, color: tone),