Fetch initial diffs asynchronously in diff stream (#1376)

Move the blocking get_diffs call into a spawned task so the WebSocket
stream is returned immediately. This prevents timeouts when fetching
diffs for repositories with many changed files.

Also remove a duplicate useEffect in DiffsPanel.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

frontend/src/components/panels/DiffsPanel.tsx

@@ -39,10 +39,6 @@ export function DiffsPanel({ selectedAttempt, gitOps }: DiffsPanelProps) {
     setHasInitialized(false);
   }, [selectedAttempt?.id]);
 
-  useEffect(() => {
-    setLoading(true);
-  }, [selectedAttempt?.id]);
-
   useEffect(() => {
     if (diffs.length > 0 && loading) {
       setLoading(false);