feat(skills): enhance error messages with i18n support

- Add structured error format with error codes and context
- Create skillErrorParser to format errors for user-friendly display
- Add comprehensive i18n keys for all skill-related errors (zh/en)
- Extend download timeout from 15s to 60s to reduce false positives
- Fix: Pass correct error title based on operation context (load/install/uninstall)

Error improvements:
- SKILL_NOT_FOUND: Show skill directory name
- DOWNLOAD_TIMEOUT: Display repo info and timeout duration with network suggestion
- DOWNLOAD_FAILED: Show HTTP status code with specific suggestions (403/404/429)
- SKILL_DIR_NOT_FOUND: Show full path with URL check suggestion
- EMPTY_ARCHIVE: Suggest checking repository URL

Users will now see detailed error messages instead of generic "Error".

src-tauri/src/error.rs

@@ -94,3 +94,28 @@ impl From<AppError> for String {
         err.to_string()
     }
 }
+
+/// 格式化为 JSON 错误字符串，前端可解析为结构化错误
+pub fn format_skill_error(
+    code: &str,
+    context: &[(&str, &str)],
+    suggestion: Option<&str>,
+) -> String {
+    use serde_json::json;
+
+    let mut ctx_map = serde_json::Map::new();
+    for (key, value) in context {
+        ctx_map.insert(key.to_string(), json!(value));
+    }
+
+    let error_obj = json!({
+        "code": code,
+        "context": ctx_map,
+        "suggestion": suggestion,
+    });
+
+    serde_json::to_string(&error_obj).unwrap_or_else(|_| {
+        // 如果 JSON 序列化失败，返回简单格式
+        format!("ERROR:{}", code)
+    })
+}