feat(merchants): implement merchant availability and settings

[Richiey1]

This PR implements end-to-end merchant availability management and configuration settings.

Key Changes:

• Backend:
  • Added and to and .
  • Implemented new routes in :
    • : Retrieve current merchant settings.
    • : Update spread, min/max trade amounts.
    • : Toggle merchant status between online (verified), offline (pending), and paused.
  • Enhanced to block trades if the merchant is not 'verified' (online).
• Frontend (Mobile):
  • Integrated into the app routing.
  • Added 'Merchant Settings' link to the page.
  • Implemented real-time availability toggling with visual feedback on the page (paused banner).
  • Connected configuration form to the new backend endpoints.

Closes #74

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