Refactor: Implement type-safe error handling in challenge room

[MohitBareja16]

Description

Closes #218

This PR eliminates the any type from the core code submission error handler in the challenge room (frontend/src/app/challenges/[id]/page.tsx).

Changes Made

• Type-Safe Catch Block: Replaced catch (error: any) with catch (error: unknown) during the handleSubmit API call.
• Error Type-Guards: Added an error instanceof Error conditional check when setting the terminal output string. This prevents runtime errors and undefined state issues if a non-standard object or primitive is thrown by the browser's fetch engine.

Verification

• Simulated a network failure during code submission.
• Verified that the custom terminal safely catches the error and outputs the fallback text without crashing the page.

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