fix: include client_secret in device code request and improve error guidance

- Include client_secret in the initial POST to /device/code endpoint
- Add check for empty GOOGLE_CLIENT_SECRET with clear error message
- Improve 'Invalid client type' error to suggest creating Desktop app OAuth client
- Include error code in addition to description for better diagnostics

src/infrastructure/api.rs

@@ -72,15 +72,25 @@ impl ApiClient {
     pub async fn authenticate(&self) -> Result<(String, String), ApiError> {
         if self.client_id.is_empty() {
             return Err(ApiError::Auth(
-                "GOOGLE_CLIENT_ID not set".to_string(),
+                "GOOGLE_CLIENT_ID not set. Set both GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET".to_string(),
+            ));
+        }
+        if self.client_secret.is_empty() {
+            return Err(ApiError::Auth(
+                "GOOGLE_CLIENT_SECRET not set. Set both GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET".to_string(),
             ));
         }
 
-        let params = serde_json::json!({
+        let mut params = serde_json::json!({
             "client_id": self.client_id,
             "scope": "https://www.googleapis.com/auth/tasks",
         });
 
+        // Some client types require client_secret in the initial request
+        if !self.client_secret.is_empty() {
+            params["client_secret"] = serde_json::Value::String(self.client_secret.clone());
+        }
+
         let resp = self
             .client
             .post("https://oauth2.googleapis.com/device/code")
@@ -96,11 +106,22 @@ impl ApiClient {
             .map_err(|e| ApiError::Api(format!("Invalid response (status {}): {}", status, e)))?;
 
         if !status.is_success() {
+            let err_code = data["error"].as_str().unwrap_or("unknown_error");
             let err_desc = data["error_description"]
                 .as_str()
-                .or_else(|| data["error"].as_str())
-                .unwrap_or("unknown error");
-            return Err(ApiError::Api(format!("OAuth error ({}): {}", status, err_desc)));
+                .unwrap_or("no description");
+
+            let mut msg = format!("OAuth error ({}): {} - {}", status, err_code, err_desc);
+
+            // Common fixes for known errors
+            if err_desc.contains("Invalid client") || err_code == "invalid_client" {
+                msg.push_str(
+                    ". Check that you created a 'Desktop app' OAuth 2.0 Client ID in Google Cloud Console \
+                     (not 'Web application'). Also verify that Google Tasks API is enabled.",
+                );
+            }
+
+            return Err(ApiError::Api(msg));
         }
 
         let url = data["verification_url"]