fix: add CSP headers to Next.js app (#127)

[FaithOnuh]

This PR resolves #127 by adding Content Security Policy (CSP) headers to the Next.js application via next.config.ts.

Changes

• Added CSP headers in apps/web/next.config.ts using Content-Security-Policy-Report-Only as requested.
• Fixed several build errors in the web app and stellar package:
  • Replaced deprecated SorobanRpc with rpc in @stellar/stellar-sdk.
  • Added missing @noble/ed25519 dependency.
  • Fixed type errors in Supabase client calls.
  • Updated @noble/ed25519 verify usage for v3 (synchronous).

Acceptance Criteria

• CSP header set via next.config.js headers()
• Policy restricts script-src to self and trusted CDNs
• Report-only mode first
• Verified build compiles successfully (type-check passes)