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/designs/Shell.dart

@@ -27,6 +27,9 @@ class Sheet extends StatelessWidget {
   /// Optional trailing control in the chrome.
   final Widget? action;
 
+  /// Space under the content; tabs leave room for the floating nav.
+  final double bottomSpace;
+
   /// Stage colour. Leaving it null (with no banner) gives the ambient tone.
   final Color? chrome;
 
@@ -53,6 +56,7 @@ class Sheet extends StatelessWidget {
     this.scrollable = true,
     this.scene,
     this.sceneSeed = 0,
+    this.bottomSpace = 40,
   });
 
   bool get _light => chrome == null && banner == null && scene == null;
@@ -62,8 +66,7 @@ class Sheet extends StatelessWidget {
     final bool light = _light;
     final Color stage = chrome ?? colorPrimaryDark;
     final Color ink = light ? colorInk : colorWhite;
-    final Color soft =
-        light ? colorInkSoft : colorWhite.withValues(alpha: 0.5);
+    final Color soft = light ? colorInkSoft : colorWhite.withValues(alpha: 0.5);
 
     final Widget header = SafeArea(
       bottom: false,
@@ -80,8 +83,7 @@ class Sheet extends StatelessWidget {
                       ? null
                       : Align(
                           alignment: Alignment.centerLeft,
-                          child: glassCircle(
-                              CupertinoIcons.arrow_left, onBack!,
+                          child: glassCircle(CupertinoIcons.arrow_left, onBack!,
                               onDark: !light),
                         ),
                 ),
@@ -136,14 +138,14 @@ class Sheet extends StatelessWidget {
         decoration: BoxDecoration(
           color: light ? colorFrost : colorPrimaryLight,
           borderRadius: const BorderRadius.only(
-            topLeft: Radius.zero,
-            topRight: Radius.zero,
+            topLeft: Radius.circular(16),
+            topRight: Radius.circular(16),
           ),
         ),
         clipBehavior: Clip.antiAlias,
         child: scrollable
             ? SingleChildScrollView(
-                padding: const EdgeInsets.fromLTRB(20, 28, 20, 40),
+                padding: EdgeInsets.fromLTRB(20, 28, 20, bottomSpace),
                 child: child,
               )
             : Padding(