Fix Error handling for ai command bar

[gajendraxdev]

currently we just checking the error response key word using .include in src/components/terminal/AiCommandBar.tsx line 360...367

fix

• set constant ( a structured error object ) instead of error being a string it looks like this

{
  "code": "AI_DISABLED",
  "message": "AI features have been disabled in your settings.",
  "provider": "ollama"
}

and frontend code look likke this

const classifyError = (err: AppError): ErrorKind => {
   if (err.code === 'AI_DISABLED') return 'disabled';
   if (err.code === 'AUTH_FAILED') return 'invalid-key';
   // No strings, no guesses!
   return 'generic';
};