Feat/mobile nav menu 13

[manueldezman]

Closes #13

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Summary

Adds a mobile navigation drawer for small screens and moves merchant-facing pages into the shared dashboard shell so the standard sidebar/topbar appears consistently across merchant routes.

Changes

• Added a mobile-only hamburger menu in the topbar with a slide-out navigation drawer.
• Shared navigation item definitions between desktop sidebar and mobile drawer.
• Hid the desktop sidebar on small screens and removed mobile left-offset spacing.
• Moved merchant routes into the (dashboard) route group while preserving public URLs.
• Added a placeholder /customers page for the existing Customers nav item.
• Fixed a JSX unescaped entities lint error in payment instructions copy.

Verification

• npm run typecheck
• npm run format:check
• npm run lint

Screenshot

Screencast.from.2026-05-31.14-21-01.mp4

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