fix(profile): upload banners without square-cropping them

[dmnyc]

Summary

Profile banner uploads came out square (pfp-shaped) because the profile editor sent banners to nostr.build's /api/v2/upload/profile endpoint, which crops to a square profile picture. The NIP-96 media_type='banner' hint added in #423-era work isn't honored by that endpoint (its advertised media_transformations.image is just resize/convert/compress — no avatar/banner cropping), so banners kept getting square-cropped.

Fix

Route banner uploads through the general /api/v2/upload/files endpoint — the same one the composer uses — which preserves the uploaded aspect ratio. Avatars stay on /upload/profile, where a square crop is the correct behavior.

const url =
  type === 'picture'
    ? 'https://nostr.build/api/v2/upload/profile'  // square pfp crop
    : 'https://nostr.build/api/v2/upload/files';    // preserves aspect ratio

The NIP-98 auth ['u', url] tag and the data[0].url response parsing already key off the same url variable, so both paths stay consistent. The avatar-only uploader in src/routes/user/[slug]/+page.svelte was checked and is unaffected.

Test plan

1. Open the profile editor and upload a wide banner image → it stays wide (no longer square-cropped).
2. Upload an avatar → still correctly square-cropped.