fix(init): show config.json path in auto-detect error

When auto_detect_db_dir() fails, the error told the user to edit
config.json without saying where that file lives. On Windows that is
%USERPROFILE%\.wx-cli\config.json, which is non-obvious.

Use the config_path already computed at the top of cmd_init() so the
error message includes the absolute path, plus a concrete example of
the db_dir shape.

src/cli/init.rs

@@ -35,8 +35,13 @@ pub fn cmd_init(force: bool) -> Result<()> {
 
     // Step 1: 检测 db_dir
     println!("检测微信数据目录...");
-    let db_dir = config::auto_detect_db_dir()
+    let db_dir = config::auto_detect_db_dir().with_context(|| format!(
-        .context("未能自动检测到微信数据目录\n请手动编辑 config.json 中的 db_dir 字段")?;
+        "未能自动检测到微信数据目录\n\
+         请编辑配置文件并填写 db_dir 字段:\n  \
+         {}\n\
+         （文件不存在则首次保存后自动创建；db_dir 示例: <data_root>\\xwechat_files\\<wxid>\\db_storage）",
+        config_path.display()
+    ))?;
     println!("找到数据目录: {}", db_dir.display());
 
     // Step 2: 扫描密钥（需要 root/sudo）