实现 AppleHandler 和 SamsungHandler 类，处理特定于供应商的 HEIC 特性，添加查找嵌入视频的逻辑

handlers/base_handler.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from heic_file import HEICFile
+
+class VendorHandler:
+    """
+    Abstract base class for vendor-specific logic.
+    """
+    def find_motion_photo_offset(self, heic_file: HEICFile) -> int | None:
+        """
+        Tries to find the byte offset of the embedded motion photo video.
+        Returns the offset if found, otherwise None.
+        """
+        # Default behavior: no motion photo found.
+        return None