Add image decryption and inline preview for WeChat V2 format

Support all three .dat encryption formats:
- Old XOR format: single-byte XOR, auto-detect key from magic bytes
- V1 format: AES-ECB with fixed key (md5("0")[:16]) + XOR tail
- V2 format (2025-08+): AES-128-ECB + raw middle + XOR tail

New files:
- decode_image.py: unified image decryption module (XOR/V1/V2)
- find_image_key.py: extract AES key from WeChat process memory
- find_image_key_monitor.py: continuous monitoring version for key capture

monitor_web.py changes:
- Inline image preview in Web UI with async decryption
- MonitorDBCache for mtime-based DB decryption caching
- username-to-DB mapping for image resolution chain
- /img/ endpoint for serving decoded images
- SSE image_update events for real-time preview updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config.py

@@ -12,6 +12,7 @@ _DEFAULT = {
     "db_dir": r"D:\xwechat_files\your_wxid\db_storage",
     "keys_file": "all_keys.json",
     "decrypted_dir": "decrypted",
+    "decoded_image_dir": "decoded_images",
     "wechat_process": "Weixin.exe",
 }
 
@@ -29,8 +30,21 @@ def load_config():
 
     # 将相对路径转为绝对路径
     base = os.path.dirname(os.path.abspath(__file__))
-    for key in ("keys_file", "decrypted_dir"):
+    for key in ("keys_file", "decrypted_dir", "decoded_image_dir"):
         if key in cfg and not os.path.isabs(cfg[key]):
             cfg[key] = os.path.join(base, cfg[key])
 
+    # 自动推导微信数据根目录（db_dir 的上级目录）
+    # db_dir 格式: D:\xwechat_files\<wxid>\db_storage
+    # base_dir 格式: D:\xwechat_files\<wxid>
+    db_dir = cfg.get("db_dir", "")
+    if db_dir and os.path.basename(db_dir) == "db_storage":
+        cfg["wechat_base_dir"] = os.path.dirname(db_dir)
+    else:
+        cfg["wechat_base_dir"] = db_dir
+
+    # decoded_image_dir 默认值
+    if "decoded_image_dir" not in cfg:
+        cfg["decoded_image_dir"] = os.path.join(base, "decoded_images")
+
     return cfg