Add compact mobile header with hamburger menu and slide-in drawer (#218)

[Hollujay]

Summary

Adds a compact mobile header that replaces the full desktop navigation with a hamburger-triggered slide-in drawer on viewports ≤720px.

Changes

src/components/Navbar.jsx

• Added hamburger menu button with aria-expanded and aria-label for accessibility
• Added semi-transparent backdrop overlay when the mobile drawer is open
• Added a slide-in drawer (<aside> with aria-label="Mobile navigation") containing navigation links with icons, LocaleSelect, and WalletButton
• Drawer closes on: hamburger click, nav link click, backdrop click, Escape key press
• Body scroll is locked (overflow: hidden) while the drawer is open
• Desktop behavior is completely unchanged

src/components/Navbar.css

• Added .navbar-hamburger — 44×44px touch target, display: none on desktop, shown as a flex column at ≤720px
• Added .navbar-backdrop — fixed full-screen semi-transparent overlay (z-index 90)
• Added .navbar-drawer — 280px fixed panel sliding in from the right (z-index 100, CSS transition)
• Added .navbar-drawer--open — transforms to translateX(0)
• Added .navbar-drawer-link — 44px min-height touch target with icon and label
• Added .navbar-drawer-actions — bordered section at bottom for locale/wallet
• Hidden .navbar-links and .navbar-actions at ≤720px via media query

src/responsive.css

• Removed obsolete .navbar { flex-wrap: wrap } rule since nav links and actions are now hidden on mobile

test/components/Navbar.test.jsx (new)

• 9 tests covering:
  • Brand name rendering
  • Navigation links (present in both desktop navbar and drawer)
  • Active link highlighting by route
  • Hamburger button presence and aria-expanded state
  • Drawer open/close on hamburger click
  • Drawer close on nav link click
  • Drawer close on Escape key
  • Drawer close on backdrop click
  • LocaleSelect and WalletButton rendered in drawer

test/touch-targets.test.js

• Added checks for .navbar-hamburger (min-width/min-height 44px)
• Added checks for .navbar-drawer-link (min-height 44px)

src/components/Navbar.stories.jsx

• Added MemoryRouter decorator for isolated story rendering

Closes #218

[arisu6804]

Clean diff — LGTM! 💯

[arisu6804]

Clean — merged. 🎉