fix: prevent TransferEncodingError in Gemini/Claude SSE streams

Convert yield Err(...) to yield Ok(Bytes::from(...)) with properly
formatted SSE error events. Yielding Err into Body::from_stream()
causes axum/hyper to abort the HTTP response with a TransferEncoding
error instead of delivering the error as a valid SSE data frame.

Files changed:
- src-tauri/src/proxy/handlers/gemini.rs (primary fix)
- src-tauri/src/proxy/mappers/claude/mod.rs (defense-in-depth)

Author: Stranmor

src-tauri/src/proxy/handlers/gemini.rs

@@ -343,7 +343,15 @@ pub async fn handle_generate(
                             Some(Ok(b)) => b,
                             Some(Err(e)) => {
                                 error!("[Gemini-SSE] Connection error: {}", e);
-                                yield Err(format!("Stream error: {}", e));
+                                // Yield error as SSE content to prevent hyper from
+                                // aborting chunked encoding without 0\r\n\r\n
+                                let error_json = serde_json::json!({
+                                    "error": {
+                                        "message": format!("Stream error: {}", e),
+                                        "type": "stream_error"
+                                    }
+                                });
+                                yield Ok::<Bytes, String>(Bytes::from(format!("data: {}\n\n", error_json)));
                                 break;
                             }
                             None => break,

src-tauri/src/proxy/mappers/claude/mod.rs

@@ -82,7 +82,13 @@ where
                             }
                         }
                         Err(e) => {
-                            yield Err(format!("Stream error: {}", e));
+                            let error_json = serde_json::json!({
+                                "error": {
+                                    "message": format!("Stream error: {}", e),
+                                    "type": "stream_error"
+                                }
+                            });
+                            yield Ok(Bytes::from(format!("data: {}\n\n", error_json)));
                             break;
                         }
                     }