fix: surface actual error messages in GitHub OAuth login flow

The frontend was swallowing Tauri backend errors (which are strings,
not Error instances) and always showing generic "Failed to start login."
Now the actual error message is displayed. Also adds User-Agent header
to device flow requests, a clear error message for 404 responses, and
a test for the 404 case.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/github.rs

@@ -175,14 +175,22 @@ async fn github_device_flow_start_with_base(base_url: &str) -> Result<DeviceFlow
     let response = client
         .post(format!("{}/login/device/code", base_url))
         .header("Accept", "application/json")
+        .header("User-Agent", "Laputa-App")
         .form(&[("client_id", GITHUB_CLIENT_ID), ("scope", "repo")])
         .send()
         .await
         .map_err(|e| format!("Device flow request failed: {}", e))?;
 
     if !response.status().is_success() {
+        let status = response.status();
         let body = response.text().await.unwrap_or_default();
-        return Err(format!("Device flow start failed: {}", body));
+        if status.as_u16() == 404 {
+            return Err(
+                "GitHub device flow not available. The OAuth App may not have device flow enabled."
+                    .to_string(),
+            );
+        }
+        return Err(format!("Device flow start failed ({}): {}", status, body));
     }
 
     response
@@ -204,6 +212,7 @@ async fn github_device_flow_poll_with_base(
     let response = client
         .post(format!("{}/login/oauth/access_token", base_url))
         .header("Accept", "application/json")
+        .header("User-Agent", "Laputa-App")
         .form(&[
             ("client_id", GITHUB_CLIENT_ID),
             ("device_code", device_code),
@@ -794,6 +803,23 @@ mod tests {
         assert!(result.unwrap_err().contains("Device flow start failed"));
     }
 
+    #[tokio::test]
+    async fn test_github_device_flow_start_404_gives_clear_message() {
+        let mut server = mockito::Server::new_async().await;
+        let mock = server
+            .mock("POST", "/login/device/code")
+            .with_status(404)
+            .with_body(r#"{"error":"Not Found"}"#)
+            .create_async()
+            .await;
+
+        let result = github_device_flow_start_with_base(&server.url()).await;
+        mock.assert_async().await;
+        assert!(result.is_err());
+        let err = result.unwrap_err();
+        assert!(err.contains("device flow not available"));
+    }
+
     #[tokio::test]
     async fn test_github_device_flow_poll_complete() {
         let mut server = mockito::Server::new_async().await;