feat(platform-config): add global PlatformConfigContext and expose feePercent

[Oluwaseyi89]

Pull Request: Global Platform Fee Context Provider

Closes #114

📦 Summary

This PR introduces a global PlatformConfigProvider that fetches the platform configuration (including dynamic fee percent) from GET /config on app boot and makes it available to all child components via usePlatformConfig() hook, eliminating individual backend calls.

✅ Changes

• Added PlatformConfigProvider using SWR for caching and automatic refetch
• Created usePlatformConfig() hook to access { config, isLoading, error }
• Added mock GET /api/config endpoint for development
• Wrapped app with provider in layout.tsx
• Updated NavBar.tsx to demonstrate usage

📁 Files Changed

File	Change
packages/frontend/src/contexts/PlatformConfigContext.tsx	New provider with SWR fetching
packages/frontend/src/app/api/config/route.ts	Mock config endpoint
packages/frontend/src/types/config.ts	Type definitions
packages/frontend/src/app/layout.tsx	Wrapped with provider
packages/frontend/src/components/NavBar.tsx	Consumed feePercent
packages/frontend/package.json	Added swr dependency

🧪 How to Test

1. Run the frontend app
2. Fee percent should display in the navbar (or any component using usePlatformConfig())
3. No individual backend calls needed for accessing platform fee

Closes #114

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[Oluwaseyi89]

@PeterOche please, review. All checks have passed.