Audit 9b: Dynamic color (Material You) + 48dp min touch target helpers

Dynamic color opt-in on Android 12+ via expect/actual DynamicColor:
- commonMain: expect isDynamicColorSupported() + rememberDynamicColorScheme()
- androidMain: SDK>=31 gate, dynamicLight/DarkColorScheme(context)
- iOS/JVM/JS/WASM: no-op actuals
- ThemeManager gains useDynamicColor StateFlow, persisted via ThemeStorage
- App.kt wires both currentTheme + useDynamicColor into HoneyDueTheme
- ThemeSelectionScreen exposes the "Use system colors" toggle

Touch target helpers:
- Modifier.minTouchTarget(48.dp) + Modifier.clickableWithRipple
- Applied at audit-flagged sites in CompleteTaskScreen

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composeApp/src/androidMain/kotlin/com/tt/honeyDue/storage/ThemeStorageManager.android.kt

@@ -21,9 +21,18 @@ actual class ThemeStorageManager(context: Context) {
         prefs.edit().remove(KEY_THEME_ID).apply()
     }
 
+    actual fun saveUseDynamicColor(enabled: Boolean) {
+        prefs.edit().putBoolean(KEY_USE_DYNAMIC_COLOR, enabled).apply()
+    }
+
+    actual fun getUseDynamicColor(): Boolean {
+        return prefs.getBoolean(KEY_USE_DYNAMIC_COLOR, false)
+    }
+
     companion object {
         private const val PREFS_NAME = "honeydue_theme_prefs"
         private const val KEY_THEME_ID = "theme_id"
+        private const val KEY_USE_DYNAMIC_COLOR = "use_dynamic_color"
 
         @Volatile
         private var instance: ThemeStorageManager? = null

composeApp/src/androidMain/kotlin/com/tt/honeyDue/ui/theme/DynamicColor.android.kt

@@ -0,0 +1,22 @@
+package com.tt.honeyDue.ui.theme
+
+import android.os.Build
+import androidx.compose.material3.ColorScheme
+import androidx.compose.material3.dynamicDarkColorScheme
+import androidx.compose.material3.dynamicLightColorScheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.platform.LocalContext
+
+/**
+ * Android actual: dynamic color (Material You) is available on Android 12+
+ * (API 31, [Build.VERSION_CODES.S]).
+ */
+actual fun isDynamicColorSupported(): Boolean =
+    Build.VERSION.SDK_INT >= Build.VERSION_CODES.S
+
+@Composable
+actual fun rememberDynamicColorScheme(darkTheme: Boolean): ColorScheme? {
+    if (!isDynamicColorSupported()) return null
+    val context = LocalContext.current
+    return if (darkTheme) dynamicDarkColorScheme(context) else dynamicLightColorScheme(context)
+}