ui: make start button in calibration sheet fully circular

lib/features/sensors/calibration_sheet.dart

@@ -806,10 +806,12 @@ class _RingAreaState extends State<_RingArea> with TickerProviderStateMixin {
               ],
             )
           else if (widget.onStart != null)
-            _AsyncButton(
+            Positioned.fill(
+              child: _AsyncButton(
                 onPressed: widget.onStart!,
                 label: 'Start',
-              compact: true,
+                circular: true,
+              ),
             ),
         ],
       ),
@@ -952,6 +954,7 @@ class _AsyncButton extends StatefulWidget {
     required this.label,
     this.icon,
     this.compact = false,
+    this.circular = false,
     this.enabled = true,
   });
 
@@ -959,6 +962,7 @@ class _AsyncButton extends StatefulWidget {
   final String label;
   final IconData? icon;
   final bool compact;
+  final bool circular;
   final bool enabled;
 
   @override
@@ -992,6 +996,22 @@ class _AsyncButtonState extends State<_AsyncButton> {
 
     final canPress = !_loading && widget.enabled;
 
+    if (widget.circular) {
+      final cs = Theme.of(context).colorScheme;
+      return SizedBox.expand(
+        child: TextButton(
+          style: TextButton.styleFrom(
+            shape: const CircleBorder(),
+            foregroundColor: cs.primary,
+            overlayColor: cs.onSurface,
+            padding: EdgeInsets.zero,
+          ),
+          onPressed: canPress ? _run : null,
+          child: child,
+        ),
+      );
+    }
+
     if (widget.compact) {
       return TextButton(
         style: TextButton.styleFrom(shape: shape, textStyle: const TextStyle(fontSize: 36, color: Colors.black)),