feat(auth): store JWT in HTTP-only cookie instead of JS-accessible cookie

Prevent XSS-based token theft by making the JWT inaccessible to
JavaScript. The backend now sets/clears the cookie via Set-Cookie
headers, and the frontend retrieves user info from /api/me instead
of decoding the JWT directly.

- Add JWTCookieMiddleware to parse cookie and inject claims into context
- Add /me and /logout endpoints to OpenAPI spec and handlers
- Update PostLogin to return user object + Set-Cookie header
- Replace Authorization header auth with cookie-based auth throughout
- Rewrite frontend auth to use /api/me instead of jwt-decode
- Remove jwt-decode dependency
- Configure CORS with credentials for local dev

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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