Add search and filter functionality to student exercises page

[Nicolapps]

Problem: Students see all exercises grouped by week with no way to search or filter. For courses with many weeks and exercises, finding a specific exercise or viewing only incomplete ones requires scrolling through the entire page.

Proposed Solution

• Add a search bar to filter exercises by name
• Add filter toggles: "Show incomplete only", "Show current week only"
• Client-side filtering from already-loaded data should suffice