feat(sample): make example cards transparent with configurable background

Add LocalExampleCardBackgroundColor CompositionLocal to ExampleCard so
the sample app can provide transparent backgrounds at the root level.
When transparent, border is also hidden.

Author: kdroidFilter

sample/src/commonMain/kotlin/io/github/kdroidfilter/nucleus/ui/apple/macos/sample/App.kt

@@ -79,7 +79,10 @@ import com.composables.icons.lucide.TextAlignStart
 import com.composables.icons.lucide.TextCursorInput
 import com.composables.icons.lucide.ToggleLeft
 import com.composables.icons.lucide.TriangleAlert
+import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.ui.graphics.Color
 import io.github.kdroidfilter.nucleus.ui.apple.macos.components.ColumnVisibility
+import io.github.kdroidfilter.nucleus.ui.apple.macos.sample.gallery.LocalExampleCardBackgroundColor
 import io.github.kdroidfilter.nucleus.ui.apple.macos.components.ColumnWidth
 import io.github.kdroidfilter.nucleus.ui.apple.macos.components.NavigationButtons
 import io.github.kdroidfilter.nucleus.ui.apple.macos.components.Popover
@@ -329,6 +332,7 @@ fun App() {
     }
 
     MacosTheme(darkTheme = isDark, accentColor = accentColor, colorScheme = colorScheme, glassType = glassType) {
+        CompositionLocalProvider(LocalExampleCardBackgroundColor provides Color.Transparent) {
         val toastState = rememberToastState()
 
         // Navigation state — back stack + forward stack for browser-like history
@@ -583,7 +587,7 @@ fun App() {
                 ToastHost(state = toastState)
             }
         }
-    }
+    }}
 }
 
 @Composable

sample/src/commonMain/kotlin/io/github/kdroidfilter/nucleus/ui/apple/macos/sample/gallery/ExampleCard.kt

@@ -15,18 +15,22 @@ import androidx.compose.foundation.layout.defaultMinSize
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.compositionLocalOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import io.github.kdroidfilter.nucleus.ui.apple.macos.components.Text
 import io.github.kdroidfilter.nucleus.ui.apple.macos.theme.MacosTheme
 
+val LocalExampleCardBackgroundColor = compositionLocalOf<Color?> { null }
+
 private enum class ExampleTab { Preview, Code }
 
 @Composable
@@ -43,11 +47,19 @@ fun ExampleCard(
             Modifier
                 .fillMaxWidth()
                 .clip(MacosTheme.shapes.large)
-                .border(
-                    width = 1.dp,
-                    color = MacosTheme.colorScheme.border,
-                    shape = MacosTheme.shapes.large,
-                ).background(MacosTheme.colorScheme.card),
+                .then(
+                    if (LocalExampleCardBackgroundColor.current != null) {
+                        Modifier.background(LocalExampleCardBackgroundColor.current!!)
+                    } else {
+                        Modifier
+                            .border(
+                                width = 1.dp,
+                                color = MacosTheme.colorScheme.border,
+                                shape = MacosTheme.shapes.large,
+                            )
+                            .background(MacosTheme.colorScheme.card)
+                    }
+                ),
     ) {
         // Header: title + description + tabs
         Column(