fix: improve invalid_client error message with setup steps

- Remove client_secret from initial /device/code request (desktop app type)
- Update error message with full Google Cloud Console setup checklist:
  1. Enable Google Tasks API
  2. Configure OAuth consent screen (Testing mode + test users + scopes)
  3. Create Desktop app OAuth client
- Copy credentials exactly without extra whitespace

src/infrastructure/api.rs

@@ -81,16 +81,11 @@ impl ApiClient {
             ));
         }
 
-        let mut params = serde_json::json!({
+        let 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")
@@ -113,11 +108,17 @@ impl ApiClient {
 
             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" {
+            if err_code == "invalid_client" || err_desc.contains("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.",
+                    "\n\nPossible fixes (Google Cloud Console):\n\
+                     1. APIs & Services > Library -> Enable 'Google Tasks API'.\n\
+                     2. APIs & Services > OAuth consent screen:\n\
+                        - Set 'Publishing status' to 'Testing'\n\
+                        - Add 'https://www.googleapis.com/auth/tasks' to Scopes\n\
+                        - Add your email under 'Test users'\n\
+                     3. APIs & Services > Credentials:\n\
+                        - Create new OAuth 2.0 Client ID of type 'Desktop app'\n\
+                        - Copy the Client ID and Client Secret exactly (no extra spaces)",
                 );
             }