Add other security headers (X-Frame-Options, Referrer-Policy, etc.)

[Marvell69]

Description
Beyond CSP (Issue 116), standard security headers (X-Frame-Options or frame-ancestors via CSP, Referrer-Policy, X-Content-Type-Options, Permissions-Policy) should be verified/added — a financial dApp is a reasonable clickjacking target absent explicit frame protection.

Requirements and context

• Add the standard security header set via next.config.ts headers configuration
• Verify via an online header-scanning tool against a preview deployment
• Document in docs/security-headers.md (companion to Issue 116)

Suggested execution

git checkout -b feat/add-standard-security-headers

• Add security headers configuration
• Verify against a deployed preview
• Document

Example commit message
feat: add standard security headers (X-Frame-Options, Referrer-Policy, etc.)