feat: add Linux support with cross-platform memory scanning

- Add Linux memory scanner (`find_all_keys_linux.py`) using `/proc/<pid>/mem`,
  same approach as Windows/macOS — no GDB, no function offsets, no restart needed
- Extract Windows-specific code to `find_all_keys_windows.py`
- Make `find_all_keys.py` a platform dispatcher (Windows / Linux)
- Add `key_utils.py` for cross-platform path matching (`/` vs `\` in all_keys.json)
- Update `config.py` with Linux auto-detection of db_storage paths
- Update all consumers (decrypt_db, monitor, monitor_web, mcp_server) to use
  `get_key_info()` for platform-agnostic key lookup

Tested on remote Linux container: 15/15 DBs scanned, decrypted, and verified.

config.example.json

@@ -1,6 +1,6 @@
-{
-    "db_dir": "D:\\xwechat_files\\your_wxid\\db_storage",
-    "keys_file": "all_keys.json",
-    "decrypted_dir": "decrypted",
-    "wechat_process": "Weixin.exe"
-}
+{
+    "db_dir": "D:\\xwechat_files\\your_wxid\\db_storage",
+    "keys_file": "all_keys.json",
+    "decrypted_dir": "decrypted",
+    "wechat_process": "Weixin.exe"
+}