feat: Improve UX with loading states, empty states, and feedback

[Nicolapps]

Problem: Several pages render blank (return null) during redirects or data loading. There are no empty state messages when lists have no items, skeleton/loading patterns are inconsistently implemented inline

Proposed Solution

• Add a shared <FullPageLoader> for redirect pages (root, login, course redirect)
• Add <EmptyState> components for empty lists (exercises, lectures, reports)
• Create a reusable <Skeleton> component to replace ad-hoc inline loading states
• Add breadcrumb navigation on deep pages (exercises, attempts)