fix: 修复饼图在浅色模式下图例文字颜色显示问题

- 使用 Material Theme 的 onSurface 颜色来设置图例文字颜色
- 确保文字颜色正确跟随系统主题
- 优化代码结构和注释

app/src/main/java/com/yovinchen/bookkeeping/ui/components/CategoryPieChart.kt

@@ -1,13 +1,17 @@
 package com.yovinchen.bookkeeping.ui.components
 
-import android.graphics.Color
+import android.graphics.Color as AndroidColor
+import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.height
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.toArgb
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.viewinterop.AndroidView
 import com.github.mikephil.charting.charts.PieChart
+import com.github.mikephil.charting.components.Legend
 import com.github.mikephil.charting.data.PieData
 import com.github.mikephil.charting.data.PieDataSet
 import com.github.mikephil.charting.data.PieEntry
@@ -19,6 +23,9 @@ fun CategoryPieChart(
     categoryData: List<Pair<String, Float>>,
     modifier: Modifier = Modifier
 ) {
+    val isDarkTheme = isSystemInDarkTheme()
+    val textColor = MaterialTheme.colorScheme.onSurface.toArgb()
+
     AndroidView(
         modifier = modifier
             .fillMaxWidth()
@@ -28,11 +35,29 @@ fun CategoryPieChart(
                 description.isEnabled = false
                 setUsePercentValues(true)
                 setDrawEntryLabels(true)
-                legend.isEnabled = true
+                
+                // 配置图例
+                legend.apply {
+                    isEnabled = true
+                    this.textColor = textColor  // 使用Material Theme的文字颜色
+                    textSize = 12f
+                    form = Legend.LegendForm.CIRCLE
+                    formSize = 12f
+                    formToTextSpace = 8f
+                    xEntrySpace = 16f
+                }
+                
                 isDrawHoleEnabled = true
                 holeRadius = 40f
-                setHoleColor(Color.TRANSPARENT)
+                setHoleColor(AndroidColor.TRANSPARENT)
                 setTransparentCircleRadius(45f)
+                
+                // 设置标签文字颜色为白色（因为标签在彩色扇形上）
+                setEntryLabelColor(AndroidColor.WHITE)
+                setEntryLabelTextSize(12f)
+                
+                // 设置中心文字颜色跟随主题
+                setCenterTextColor(textColor)
             }
         },
         update = { chart ->
@@ -44,7 +69,7 @@ fun CategoryPieChart(
                 colors = ColorTemplate.MATERIAL_COLORS.toList()
                 valueTextSize = 14f
                 valueFormatter = PercentFormatter(chart)
-                valueTextColor = Color.WHITE
+                valueTextColor = AndroidColor.WHITE  // 扇形上的数值文字保持白色
                 setDrawValues(true)
             }