chore: deep semantic HTML audit and improvements across all pages

components/Header.tsx

@@ -341,7 +341,7 @@ export default function Header() {
           aria-label={t('menu')}
           ref={mobileMenuRef}
         >
-          <motion.div
+          <motion.nav
             className="flex-grow flex flex-col justify-center items-center p-8 space-y-8"
             initial="closed"
             animate={isMobileMenuOpen ? 'open' : 'closed'}
@@ -463,7 +463,7 @@ export default function Header() {
                 <Image src="/logo-white.svg" alt={t('home')} width={80} height={80} unoptimized />
               </motion.div>
             </motion.div>
-          </motion.div>
+          </motion.nav>
         </div>
       </motion.header>
     </>