feat: add trustProxy option to respect X-Forwarded-* headers

[bartlomieju]

Summary

• Adds trustProxy option to FreshConfig (default: false)
• When enabled, the handler reads X-Forwarded-Proto and X-Forwarded-Host headers and rewrites ctx.url so it reflects the actual client-facing URL
• Useful for apps running behind nginx, Caddy, or other reverse proxies where ctx.url would otherwise return http:// instead of https://

Closes #2660

Usage

const app = new App({ trustProxy: true });

Test plan

• With trustProxy: true, send a request with X-Forwarded-Proto: https and X-Forwarded-Host: example.com — verify ctx.url reflects https://example.com/...
• With trustProxy: false (default), same headers are ignored — ctx.url uses the original request URL
• Type checks pass

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