fix(streaming): detect premature connection close and surface error to user

When a reverse proxy (e.g., Tailscale Serve, nginx) closes the SSE
connection after an idle timeout, the reader returns { done: true }
cleanly. Previously this was treated as a successful completion, causing
the thinking indicator to disappear and a retry button to appear with
no error message — silently discarding any in-progress backend work.

Now: if the stream ends without a 'done' event AND no response text was
received while in 'accepted' or 'active' phase, call markFailed() with
a clear message instead of finishStream(). This surfaces an error toast
and persists the error state so the user knows what happened.

Fixes #512.

Validation: pnpm build passes, use-streaming-message tests (4/4) pass.

src/screens/chat/hooks/use-streaming-message.ts

@@ -983,7 +983,21 @@ export function useStreamingMessage(options: UseStreamingMessageOptions = {}) {
 
         const lifecyclePhase = lifecyclePhaseRef.current as StreamLifecyclePhase
         if (!finishedRef.current && lifecyclePhase !== 'handoff') {
-          finishStream()
+          // If the stream ended cleanly (no 'done' event) but we never received
+          // any response text, treat it as a failure rather than a successful
+          // empty completion. This happens when a proxy (e.g., Tailscale Serve)
+          // closes the connection after an idle timeout — the reader returns
+          // { done: true } but the model was still generating. Fixes #512.
+          if (
+            !fullTextRef.current &&
+            (lifecyclePhase === 'accepted' || lifecyclePhase === 'active')
+          ) {
+            markFailed(
+              'Connection closed before response was received. The backend may still be processing — check server logs or retry.',
+            )
+          } else {
+            finishStream()
+          }
         }
       } catch (err) {
         if ((err as Error).name === 'AbortError') {