The app shell's only navigation was the desktop sidebar (`hidden … sm:flex`), so
on phones (< sm) it was hidden with no replacement — you couldn't navigate at all.
Add a responsive mobile layout shown only below `sm`: a top bar (brand + language
switcher) and a fixed bottom tab bar with the five nav items (icon + label). The
desktop sidebar is unchanged. Page content gets bottom padding so the fixed bar
doesn't cover it, and the bar respects the iOS `safe-area-inset-bottom`.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
a46abb1b56