Refactor: Enforce strict Problem typings in problems/page.tsx

[MohitBareja16]

Description

Closes #216

This PR hardens the type safety of the problem library page (frontend/src/app/problems/page.tsx) by removing any types during data iteration.

Changes Made

• Interface Definition: Created a strict Problem interface mapping to the expected backend schema (tracking _id, slug, title, difficulty, and category).
• Strict Iteration: Replaced the generic problems.map((problem: any) with problems.map((problem: Problem). This guarantees that the UI rendering logic strictly adheres to the defined schema and will throw compiler errors if properties are manipulated incorrectly.

Verification

• The Next.js build step successfully passes type-checking for this page.
• The problem list renders correctly with accurate titles, difficulty tags, and categories.

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