Switch to Inter, round the corners softly and make the nav switch tabs

Inter replaces General Sans (Inter Display for titles and big numbers).
Corners come back from before the squaring commit, scaled to a soft set
(16 cards, 14 buttons, 12 rows, 8 chips) and applied to the screens built
since. The nav is now a cluster of round frosted buttons that switches
Today, Routines, Goals, Groups and Progress in place inside Home: no
pushed pages, no back arrows, and Android back returns to Today.

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

frontend/lib/Grounded/see/login/LoginState.dart

@@ -107,8 +107,8 @@ class LoginState extends State<Login> implements ConnectLogin {
             decoration: BoxDecoration(
               color: colorPrimaryLight,
               borderRadius: const BorderRadius.only(
-                topLeft: Radius.zero,
-                topRight: Radius.zero,
+                topLeft: Radius.circular(16),
+                topRight: Radius.circular(16),
               ),
             ),
             clipBehavior: Clip.antiAlias,