feat: 添加深色模式支持，优化图片查看器滑动切换

android/app/src/main/java/com/memory/app/data/ThemeManager.kt

@@ -0,0 +1,36 @@
+package com.memory.app.data
+
+import android.content.Context
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+
+enum class ThemeMode {
+    LIGHT, DARK, SYSTEM
+}
+
+class ThemeManager(context: Context) {
+    private val prefs = context.getSharedPreferences("theme_prefs", Context.MODE_PRIVATE)
+    
+    private val _themeMode = MutableStateFlow(loadThemeMode())
+    val themeMode: StateFlow<ThemeMode> = _themeMode.asStateFlow()
+    
+    private fun loadThemeMode(): ThemeMode {
+        val value = prefs.getString("theme_mode", ThemeMode.LIGHT.name)
+        return try {
+            ThemeMode.valueOf(value ?: ThemeMode.LIGHT.name)
+        } catch (e: Exception) {
+            ThemeMode.LIGHT
+        }
+    }
+    
+    fun setThemeMode(mode: ThemeMode) {
+        prefs.edit().putString("theme_mode", mode.name).apply()
+        _themeMode.value = mode
+    }
+    
+    fun toggleTheme() {
+        val newMode = if (_themeMode.value == ThemeMode.DARK) ThemeMode.LIGHT else ThemeMode.DARK
+        setThemeMode(newMode)
+    }
+}