security: add CSP and security headers to middleware

[isaJrKai]

What this fixes

The app had no Content-Security-Policy header, meaning an XSS vulnerability anywhere in the app (now or in the future) could execute arbitrary scripts, steal cookies, or exfiltrate data.

Headers added

Header	Value	Purpose
Content-Security-Policy	default-src 'self'; script-src 'self' 'unsafe-inline'; ...	Prevents XSS, clickjacking, data exfiltration
X-Frame-Options	DENY	Clickjacking defense (legacy browsers)
X-Content-Type-Options	nosniff	Prevents MIME sniffing
Referrer-Policy	strict-origin-when-cross-origin	Limits referrer leakage
X-XSS-Protection	1; mode=block	Legacy XSS filter

CSP policy details

default-src 'self';
script-src 'self' 'unsafe-inline';
style-src 'self' 'unsafe-inline';
img-src 'self' data: blob:;
font-src 'self' data:;
connect-src 'self';
frame-ancestors 'none';
base-uri 'self';
form-action 'self';

• script-src 'unsafe-inline' — required for Next.js hydration. Future improvement: use nonce-based CSP.
• style-src 'unsafe-inline' — required for Next.js + styled-jsx.
• img-src data: blob: — allows data-URI avatars and blob uploads.
• connect-src 'self' — blocks external API calls (no exfiltration).
• frame-ancestors 'none' — prevents the app from being iframed.

Files changed

• src/middleware.ts — added applySecurityHeaders() helper, applied to all response paths

Verification

• ✅ npx tsc --noEmit — clean
• ✅ npx eslint — clean

Vercel env vars

None needed.

Deployment risk

Low. Security headers are additive — they don't change app behavior, only add headers to responses. If the CSP is too strict, the browser console will show CSP violations (check DevTools → Console after deploy). The policy is permissive enough for Next.js's needs.

Testing after merge

1. Open the deployed app
2. Open DevTools → Console
3. Look for red CSP violation errors
4. If any appear, they'll indicate which directive needs loosening

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 501abc9f-bef3-467c-ad4d-c3abfa0914e5

📥 Commits

Reviewing files that changed from the base of the PR and between 7053878 and d978de3.

📒 Files selected for processing (1)

• src/middleware.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/csp-header

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