Fixes #1164

Added input validation and error handling for the Calculator/saikrishna823 project.

Changes:

• Shows "Invalid input" for empty or malformed expressions.
• Shows "Cannot divide by zero" for division by zero.
• Clears error state when users enter a new value or press clear.
• Adds a small error style for invalid results.

Tested:

• 1+2 returns 3
• Empty input shows Invalid input
• 1++2 shows Invalid input
• 5/0 shows Cannot divide by zero
• Clear button resets the display