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/main.dart

@@ -48,7 +48,7 @@ class GroundedApp extends StatelessWidget {
             fontSize: 17,
           ),
           shape: RoundedRectangleBorder(
-            borderRadius: BorderRadius.circular(20),
+            borderRadius: BorderRadius.zero,
           ),
         ),
       ),