fix: add explicit white color to default subtitle text

The default subtitle text in all control widgets (Cupertino, Material,
and Material Desktop) did not specify an explicit text color. This could
cause subtitles to be invisible on certain themes where the default text
color is dark, as subtitles are rendered on a semi-transparent black
background.

This fix adds explicit `color: Colors.white` to ensure subtitle text is
always visible regardless of the app's theme configuration.

Addresses #914

Author: nathannewyen

lib/src/cupertino/cupertino_controls.dart

@@ -222,7 +222,10 @@ class _CupertinoControlsState extends State<CupertinoControls>
         ),
         child: Text(
           currentSubtitle.first!.text.toString(),
-          style: const TextStyle(fontSize: 18),
+          style: const TextStyle(
+            fontSize: 18,
+            color: Colors.white,
+          ),
           textAlign: TextAlign.center,
         ),
       ),

lib/src/material/material_controls.dart

@@ -234,7 +234,10 @@ class _MaterialControlsState extends State<MaterialControls>
         ),
         child: Text(
           currentSubtitle.first!.text.toString(),
-          style: const TextStyle(fontSize: 18),
+          style: const TextStyle(
+            fontSize: 18,
+            color: Colors.white,
+          ),
           textAlign: TextAlign.center,
         ),
       ),

lib/src/material/material_desktop_controls.dart

@@ -247,7 +247,10 @@ class _MaterialDesktopControlsState extends State<MaterialDesktopControls>
         ),
         child: Text(
           currentSubtitle.first!.text.toString(),
-          style: const TextStyle(fontSize: 18),
+          style: const TextStyle(
+            fontSize: 18,
+            color: Colors.white,
+          ),
           textAlign: TextAlign.center,
         ),
       ),