Add error message when Python is not installed

[Copilot]

Problem

When users try to run Python files using the run button but don't have Python installed on their system, nothing happens - the run button silently fails with no feedback to the user.

Solution

Added proper error handling when Python executable is not found:

1. Created a utility function to check if an executable exists in the system PATH
2. Added error handling in both terminal and background execution code paths
3. Show a clear error message when Python is not found with an "Install Python" button

Testing

Verified that:

• When Python is installed, the run functionality continues to work as before
• When Python is not installed, a clear error message is shown instead of silently failing

Screenshots

When Python is not found, the user now sees:

Fixes #391.

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