feat: 新增设计系统（颜色/字体/浅色主题）

app/src/main/java/com/placeholder/ringhealth/MainActivity.kt

@@ -12,6 +12,7 @@ import androidx.compose.material3.Text
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.res.stringResource
+import com.placeholder.ringhealth.core.designsystem.RingHealthTheme
 
 /** 唯一 Activity，承载 Compose 界面。 */
 class MainActivity : ComponentActivity() {
@@ -19,8 +20,11 @@ class MainActivity : ComponentActivity() {
         super.onCreate(savedInstanceState)
         enableEdgeToEdge()
         setContent {
-            MaterialTheme {
+            RingHealthTheme {
-                Surface(modifier = Modifier.fillMaxSize()) {
+                Surface(
+                    modifier = Modifier.fillMaxSize(),
+                    color = MaterialTheme.colorScheme.background,
+                ) {
                     Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
                         // 文案取自 res/values/strings.xml，不在 Kotlin 里硬编码用户可见文字
                         Text(stringResource(R.string.app_name))

app/src/main/java/com/placeholder/ringhealth/core/designsystem/Color.kt

@@ -0,0 +1,23 @@
+package com.placeholder.ringhealth.core.designsystem
+
+import androidx.compose.ui.graphics.Color
+
+/** 品牌主色（健康绿，仿小米运动健康首页大环）。 */
+val BrandGreen = Color(0xFF34C759)
+val BrandGreenDark = Color(0xFF248A3D)
+
+/** 页面与卡片背景。 */
+val AppBackground = Color(0xFFF5F6F8)
+val CardSurface = Color(0xFFFFFFFF)
+val TextPrimary = Color(0xFF1C1C1E)
+val TextSecondary = Color(0xFF8E8E93)
+val DividerColor = Color(0xFFEFEFF4)
+
+/** 各健康指标的语义色，全应用统一引用。 */
+val MetricGreen = Color(0xFF34C759)      // 步数
+val MetricRed = Color(0xFFFF3B30)        // 心率
+val MetricBlue = Color(0xFF3B5BFF)       // 睡眠
+val MetricCyan = Color(0xFF00C7BE)       // 血氧
+val MetricPurple = Color(0xFFAF52DE)     // 压力
+val MetricOrange = Color(0xFFFF9500)     // 体温
+val MetricYellow = Color(0xFFFFCC00)     // 血压 / 提醒

app/src/main/java/com/placeholder/ringhealth/core/designsystem/Theme.kt

@@ -0,0 +1,28 @@
+package com.placeholder.ringhealth.core.designsystem
+
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.lightColorScheme
+import androidx.compose.runtime.Composable
+
+private val RingHealthColors = lightColorScheme(
+    primary = BrandGreen,
+    onPrimary = androidx.compose.ui.graphics.Color.White,
+    secondary = BrandGreenDark,
+    background = AppBackground,
+    onBackground = TextPrimary,
+    surface = CardSurface,
+    onSurface = TextPrimary,
+    surfaceVariant = DividerColor,
+)
+
+/**
+ * 应用主题。阶段一仅提供浅色模式（深色留到 M5）。
+ */
+@Composable
+fun RingHealthTheme(content: @Composable () -> Unit) {
+    MaterialTheme(
+        colorScheme = RingHealthColors,
+        typography = RingHealthTypography,
+        content = content,
+    )
+}

app/src/main/java/com/placeholder/ringhealth/core/designsystem/Type.kt

@@ -0,0 +1,23 @@
+package com.placeholder.ringhealth.core.designsystem
+
+import androidx.compose.material3.Typography
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.sp
+
+/** 大号数值样式：首页大环与详情页主数字。 */
+val MetricValueStyle = TextStyle(
+    fontSize = 40.sp,
+    fontWeight = FontWeight.Bold,
+    color = TextPrimary,
+)
+
+/** 中等数值样式：指标卡主数字。 */
+val MetricTileValueStyle = TextStyle(
+    fontSize = 22.sp,
+    fontWeight = FontWeight.SemiBold,
+    color = TextPrimary,
+)
+
+/** 全局字体排版。 */
+val RingHealthTypography = Typography()