feat: replace mobile hamburger menu with fixed bottom tab bar (#25)

Removes the dropdown mobile nav in favour of a persistent bottom bar,
matching the native app pattern users expect on mobile. Also preserves
the GitHub link added upstream and swaps the deprecated Github lucide
icon for an inline SVG.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: m4cd4r4