feat: Add authentication error handling to UI flow

Extend auth error detection to the web UI flow:

server/main.py:
- Fix setup_status() endpoint to check ~/.claude directory instead of
  non-existent .credentials.json file
- Add explanatory comments about Claude CLI credential storage changes

server/services/process_manager.py:
- Add AUTH_ERROR_PATTERNS for detecting auth errors in agent output
- Add is_auth_error() helper function
- Add AUTH_ERROR_HELP message template
- Update _stream_output() to detect auth errors in real-time
- Buffer last 20 lines to catch auth errors on process exit
- Broadcast clear help message to WebSocket clients when auth fails

start_ui.sh:
- Add Claude CLI installation check with helpful guidance
- Add ~/.claude directory check with login reminder
- Non-blocking warnings that don't prevent UI from starting

This ensures users get clear, actionable feedback when authentication
fails, whether using the CLI or the web UI.

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

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

server/main.py

@@ -120,9 +120,11 @@ async def setup_status():
     # Check for Claude CLI
     claude_cli = shutil.which("claude") is not None
 
-    # Check for credentials file
-    credentials_path = Path.home() / ".claude" / ".credentials.json"
-    credentials = credentials_path.exists()
+    # Check for Claude CLI configuration directory
+    # Note: Claude CLI no longer stores credentials in ~/.claude/.credentials.json
+    # The existence of ~/.claude indicates the CLI has been configured
+    claude_dir = Path.home() / ".claude"
+    credentials = claude_dir.exists() and claude_dir.is_dir()
 
     # Check for Node.js and npm
     node = shutil.which("node") is not None