feat: 添加CropBackgroundUtils工具类及封面剪裁背景修改功能

- 创建CropBackgroundUtils工具类，实现与BackgroundUtils类似的背景管理功能
- 在封面剪裁窗口(CropActivity)添加"修改剪裁背景颜色"按钮
- 按钮点击后弹出Toast提示信息
- 使用独立的偏好设置存储(crop_background_prefs)

gallery/src/main/java/cc/winboll/studio/gallery/CropActivity.java

@@ -87,6 +87,13 @@ public class CropActivity extends AppCompatActivity {
             }
         });
         
+        findViewById(R.id.btn_change_bg).setOnClickListener(new View.OnClickListener() {
+            @Override
+            public void onClick(View v) {
+                Toast.makeText(CropActivity.this, "修改剪裁背景颜色", Toast.LENGTH_SHORT).show();
+            }
+        });
+        
         zoomContainer = findViewById(R.id.zoom_container);
         
         SeekBar seekBarZoom = findViewById(R.id.seekbar_zoom);

gallery/src/main/java/cc/winboll/studio/gallery/utils/CropBackgroundUtils.java

@@ -0,0 +1,124 @@
+package cc.winboll.studio.gallery.utils;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import android.graphics.Color;
+import android.graphics.drawable.ColorDrawable;
+import android.graphics.drawable.Drawable;
+import androidx.annotation.ColorInt;
+import androidx.annotation.DrawableRes;
+import androidx.core.content.ContextCompat;
+
+public class CropBackgroundUtils {
+
+    public enum DrawableType {
+        RESOURCE_ID,
+        COLOR
+    }
+
+    private static volatile CropBackgroundUtils instance;
+
+    private static final String PREF_NAME = "crop_background_prefs";
+    private static final String KEY_TYPE = "crop_bg_type";
+    private static final String KEY_RES_ID = "crop_bg_res_id";
+    private static final String KEY_COLOR = "crop_bg_color";
+
+    private Context context;
+    private Drawable drawable;
+    private DrawableType drawableType;
+    private int resId;
+    private int color;
+
+    private CropBackgroundUtils() {
+    }
+
+    public static CropBackgroundUtils getInstance() {
+        if (instance == null) {
+            synchronized (CropBackgroundUtils.class) {
+                if (instance == null) {
+                    instance = new CropBackgroundUtils();
+                }
+            }
+        }
+        return instance;
+    }
+
+    public static CropBackgroundUtils initFromResource(Context context, @DrawableRes int resId) {
+        synchronized (CropBackgroundUtils.class) {
+            CropBackgroundUtils utils = getInstance();
+            utils.context = context.getApplicationContext();
+            utils.drawableType = DrawableType.RESOURCE_ID;
+            utils.resId = resId;
+            utils.drawable = ContextCompat.getDrawable(utils.context, resId);
+            utils.saveToPreferences();
+            return utils;
+        }
+    }
+
+    public static CropBackgroundUtils initFromColor(Context context, @ColorInt int color) {
+        synchronized (CropBackgroundUtils.class) {
+            CropBackgroundUtils utils = getInstance();
+            utils.context = context.getApplicationContext();
+            utils.drawableType = DrawableType.COLOR;
+            utils.color = color;
+            utils.drawable = new ColorDrawable(color);
+            utils.saveToPreferences();
+            return utils;
+        }
+    }
+
+    public static CropBackgroundUtils initFromPreferences(Context context) {
+        synchronized (CropBackgroundUtils.class) {
+            Context appContext = context.getApplicationContext();
+            SharedPreferences prefs = appContext.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
+            int type = prefs.getInt(KEY_TYPE, -1);
+            if (type == 0) {
+                int resId = prefs.getInt(KEY_RES_ID, 0);
+                if (resId != 0) {
+                    return initFromResource(appContext, resId);
+                }
+            } else if (type == 1) {
+                int color = prefs.getInt(KEY_COLOR, Color.BLACK);
+                return initFromColor(appContext, color);
+            }
+            return initFromColor(appContext, 0xFF00FF00);
+        }
+    }
+
+    public Drawable getDrawable() {
+        return drawable;
+    }
+
+    public DrawableType getDrawableType() {
+        return drawableType;
+    }
+
+    public int getResId() {
+        return resId;
+    }
+
+    public int getColor() {
+        return color;
+    }
+
+    public void saveToPreferences() {
+        if (context == null) return;
+        SharedPreferences prefs = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
+        SharedPreferences.Editor editor = prefs.edit();
+        if (drawableType == DrawableType.RESOURCE_ID) {
+            editor.putInt(KEY_TYPE, 0);
+            editor.putInt(KEY_RES_ID, resId);
+            editor.remove(KEY_COLOR);
+        } else {
+            editor.putInt(KEY_TYPE, 1);
+            editor.putInt(KEY_COLOR, color);
+            editor.remove(KEY_RES_ID);
+        }
+        editor.apply();
+    }
+
+    public static void clearPreferences(Context context) {
+        SharedPreferences prefs = context.getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
+        prefs.edit().clear().apply();
+    }
+}

gallery/src/main/res/layout/activity_crop.xml

@@ -46,6 +46,15 @@
             android:src="@drawable/ic_info"
             android:background="?attr/selectableItemBackgroundBorderless"/>
 
+        <ImageView
+            android:id="@+id/btn_change_bg"
+            android:layout_width="40dp"
+            android:layout_height="40dp"
+            android:padding="5dp"
+            android:src="@drawable/ic_color_pick"
+            android:background="?attr/selectableItemBackgroundBorderless"
+            android:contentDescription="修改剪裁背景颜色"/>
+
         <ImageView
             android:id="@+id/btn_done"
             android:layout_width="40dp"