feat(v0.2): Firebase server adapter

[genu]

Part of #179. Depends on the presign + direct + download/delete endpoints being in place.

Port the existing Firebase Storage client adapter into a server adapter exported from `nuxt-upload-kit/server/firebase`.

Scope

• `FirebaseStorage({ bucket, credentials, ... })` using `firebase-admin` directly. Presigning via `getSignedUrl()`.
• Playground example.

Notes

• Use SDK's built-in retry/expiry/presign — do not wrap.
• Old client-side `PluginFirebase` stays at `nuxt-upload-kit/providers/firebase` as an escape hatch.