fix: add Connection: close to prevent keep-alive socket reuse after abort

When AbortController.abort() resets a TCP connection mid-request,
Chromium may reuse the same keep-alive socket for the next request.
Valhalla's HTTP server then returns ERR_EMPTY_RESPONSE. Forcing
Connection: close ensures each request uses a fresh TCP connection.

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

src/renderer/src/api/ors.ts

@@ -104,7 +104,7 @@ export async function fetchIsochrones(
   try {
     response = await fetch(`${VALHALLA_BASE}/isochrone`, {
       method: 'POST',
-      headers: { 'Content-Type': 'application/json' },
+      headers: { 'Content-Type': 'application/json', 'Connection': 'close' },
       body,
       signal: abortController.signal
     })