Settings Redesign & VAT Flat Rate

Added

• Settings refactor — Horizontal tab navigation replacing vertical sidebar
• Organisation ID copy — Developers can copy their org ID for API integrations
• VAT flat rate scheme — Support for UK businesses using the Flat Rate Scheme
• Inline validation — Visual checkmarks for CRN and VAT number fields
• Sticky save bar — Floating save indicator when organisation form has changes

Changed

• 2FA and password sections collapsed by default (progressive disclosure)
• Developer tab absorbed API settings functionality
• Billing tab layout cleaned up to match design mockups