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/iosMain/kotlin/com/tt/honeyDue/storage/ThemeStorageManager.ios.kt

@@ -22,8 +22,21 @@ actual class ThemeStorageManager {
         defaults.synchronize()
     }
 
+    actual fun saveUseDynamicColor(enabled: Boolean) {
+        defaults.setBool(enabled, forKey = KEY_USE_DYNAMIC_COLOR)
+        defaults.synchronize()
+    }
+
+    actual fun getUseDynamicColor(): Boolean {
+        // iOS has no Material You — always false. Still read/write so stored
+        // state survives a platform reinstall scenario and cross-platform parity
+        // APIs stay symmetric.
+        return defaults.boolForKey(KEY_USE_DYNAMIC_COLOR)
+    }
+
     companion object {
         private const val KEY_THEME_ID = "theme_id"
+        private const val KEY_USE_DYNAMIC_COLOR = "use_dynamic_color"
 
         private val instance by lazy { ThemeStorageManager() }
 

composeApp/src/iosMain/kotlin/com/tt/honeyDue/ui/theme/DynamicColor.ios.kt

@@ -0,0 +1,10 @@
+package com.tt.honeyDue.ui.theme
+
+import androidx.compose.material3.ColorScheme
+import androidx.compose.runtime.Composable
+
+/** iOS has no equivalent to Material You wallpaper-driven colors. */
+actual fun isDynamicColorSupported(): Boolean = false
+
+@Composable
+actual fun rememberDynamicColorScheme(darkTheme: Boolean): ColorScheme? = null