fix: consolidate auth error handling and fix start.bat credential check

This commit addresses issues found during review of PRs #12 and #28:

## PR #12 (Auth Error Handling) Fixes

- Create shared auth.py module with centralized AUTH_ERROR_PATTERNS,
  is_auth_error(), and print_auth_error_help() functions
- Fix start.bat to use directory check instead of outdated
  .credentials.json file check (matching start.sh behavior)
- Update process_manager.py to import from shared auth module
- Update start.py to import from shared auth module
- Update documentation comments in autonomous_agent_demo.py and
  client.py to remove references to deprecated .credentials.json

## PR #28 (Feature Management) Improvements

- Add _priority_lock threading lock to feature_mcp.py to prevent
  race conditions when multiple features are created simultaneously
- Apply lock to feature_create, feature_create_bulk, and feature_skip
- Add checkAndSendTimeoutRef cleanup in useAssistantChat.ts to
  prevent memory leaks on component unmount
- Clear currentAssistantMessageRef on response_done

## Code Quality

- All Python files pass ruff linting
- All security tests pass (91/91)
- UI passes ESLint and TypeScript compilation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/hooks/useAssistantChat.ts

@@ -43,6 +43,7 @@ export function useAssistantChat({
   const maxReconnectAttempts = 3;
   const pingIntervalRef = useRef<number | null>(null);
   const reconnectTimeoutRef = useRef<number | null>(null);
+  const checkAndSendTimeoutRef = useRef<number | null>(null);
 
   // Clean up on unmount
   useEffect(() => {
@@ -53,9 +54,13 @@ export function useAssistantChat({
       if (reconnectTimeoutRef.current) {
         clearTimeout(reconnectTimeoutRef.current);
       }
+      if (checkAndSendTimeoutRef.current) {
+        clearTimeout(checkAndSendTimeoutRef.current);
+      }
       if (wsRef.current) {
         wsRef.current.close();
       }
+      currentAssistantMessageRef.current = null;
     };
   }, []);
 
@@ -203,6 +208,7 @@ export function useAssistantChat({
 
           case "response_done": {
             setIsLoading(false);
+            currentAssistantMessageRef.current = null;
 
             // Mark current message as done streaming
             setMessages((prev) => {
@@ -251,11 +257,18 @@ export function useAssistantChat({
 
   const start = useCallback(
     (existingConversationId?: number | null) => {
+      // Clear any pending check timeout from previous call
+      if (checkAndSendTimeoutRef.current) {
+        clearTimeout(checkAndSendTimeoutRef.current);
+        checkAndSendTimeoutRef.current = null;
+      }
+
       connect();
 
       // Wait for connection then send start message
       const checkAndSend = () => {
         if (wsRef.current?.readyState === WebSocket.OPEN) {
+          checkAndSendTimeoutRef.current = null;
           setIsLoading(true);
           const payload: { type: string; conversation_id?: number } = {
             type: "start",
@@ -266,11 +279,13 @@ export function useAssistantChat({
           }
           wsRef.current.send(JSON.stringify(payload));
         } else if (wsRef.current?.readyState === WebSocket.CONNECTING) {
-          setTimeout(checkAndSend, 100);
+          checkAndSendTimeoutRef.current = window.setTimeout(checkAndSend, 100);
+        } else {
+          checkAndSendTimeoutRef.current = null;
         }
       };
 
-      setTimeout(checkAndSend, 100);
+      checkAndSendTimeoutRef.current = window.setTimeout(checkAndSend, 100);
     },
     [connect],
   );