feat: ensure i18n-based aria-labels on all icon-only buttons
Standardize aria-labels across form-components, hamburger-menu, and header
partials to use i18n keys for multilingual support. Modal close buttons now
consistently use the 'closeMenu' i18n key across all three modals, matching
the accessibility pattern used for menu and theme toggle buttons.
- form-components.html: Update modal close buttons (alert, confirm, content)
- hamburger-menu.html: Already has aria-label with i18n
- header.html: Theme toggle and hamburger menu already have aria-labels
