gateway: UX improvement: browser-only subdomain redirects

## Summary

Implement conditional subdomain gateway redirects that only execute when the client is identified as a web browser, preventing unnecessary redirects for programmatic clients and tools.

## Rationale

As discussed in https://github.com/ipfs/kubo/issues/11024#issuecomment-3400044981, the current gateway behavior performs subdomain redirects universally, which acts as a papercut in non-browser contexts when `localhost` (subdomain gw) is used instead of `127.0.0.1` (path gw):
- creates unnecessary performance overhead due to extra redirect in non-browser clients like curl, wget, handle these redirects as expected.
- and/or breaks in Windows or macOS (https://github.com/ipfs/kubo/issues/7527) or other programmatic tools that don'tsupport subdomains correctly

## Proposed Change

The gateway should detect whether the requesting client is a browser before executing subdomain redirects. This detection can be based on:
- User-Agent header analysis
- Accept headers that indicate browser requests
- Presence of browser-specific headers

Non-browser clients would receive direct responses without subdomain redirects, maintaining compatibility with existing tooling while preserving the security benefits of subdomain isolation for web browsers.

### Browser-like detection

TBD. Unsure if naive check if `User-Agent` starts with `Mozilla` is enough.
Needs reseach what is the most robust and future proof way here.
We want to keep Origin isolation in all user agents that hint to be a browser capable of rendering HTML and JS. Could be combination of `User-Agent`, `Accept` etc.