feat: implement input validation and sanitization in terminal popup

[Suke2004]

Description:
The terminal popup accepts any input — including empty strings, numbers, and special characters — which leads to unnecessary API calls and potential injection risks. Add validation and feedback to enhance UX and safety.

✅ Acceptance Criteria:

• Disallow empty input and very short/long strings.
• Sanitize user input to remove special characters.
• Accept only valid words (letters only).
• Show visual feedback (e.g., red border or message) for invalid input.
• Prevent API call if validation fails.

[adi-ray]

@BennyPerumalla @Suke2004 Love to take a dig on this issue.

[BennyPerumalla]

@adi-ray Assigned it to you, but make sure to not relay on AI very much for this issue.