fix: patch security vulnerabilities (v0.11.0)

[kk0829]

Summary

• Fix path traversal in Static() file serving and add FileFromSafeDir() for safe file access
• Fix IP spoofing by introducing TrustedProxies config with CIDR validation for X-Real-IP / X-Forwarded-For
• Add SetCookieWithConfig() with HttpOnly, Secure, SameSite support; Helmet() middleware for security headers; RedirectSafe() with host whitelist

Breaking Changes

• RemoteAddr() no longer trusts X-Real-IP / X-Forwarded-For by default — requires TrustedProxies config
• SetCookie(key, value) deprecated in favor of SetCookieWithConfig
• DefaultApp() now registers Helmet() middleware

Full Changelog

Security Fixes

• Path traversal in Static() and File() — resolved path validated against root directory
• IP address spoofing — X-Real-IP / X-Forwarded-For only trusted from configured proxies
• Debug endpoint exposure — token-based authentication via Config.DebugToken
• Template error information leakage — errors sanitized before returning to client
• Open redirect — new RedirectSafe() with host whitelist
• Content-Disposition header injection — filename escaping
• Cookie missing security attributes — new CookieConfig with HttpOnly, Secure, SameSite

New APIs

• Config.TrustedProxies []string — CIDR whitelist
• Config.DebugToken string — debug endpoint auth
• ctx.SetCookieWithConfig(CookieConfig) — secure cookie setter
• ctx.FileFromSafeDir(safeDir, path) — safe file serving
• ctx.RedirectSafe(code, url, hosts...) — safe redirect
• Helmet() middleware — security response headers

Test Coverage

• 84.6% → 96.1% (+30 test cases)