Resolve actors via the proxyUrl endpoint instead of the ap_actor cache

## Goal

When the app needs a remote **actor object**, resolve it through the actor's `proxyUrl` endpoint — the spec-defined C2S way to fetch a remote AP object via the server — instead of reading the local `ap_actor` cache.

## Why

A real C2S client resolves remote objects through `endpoints.proxyUrl`, not a server-side post-type cache: fresher data, no direct cross-origin fetch, and consistent with the event-sourcing client (Automattic/wordpress-activitypub#3427/514). The endpoint already exists.

## Current state (verified in code)

* The app reads actors from the `ap_actor` post type (`Remote_Actors`) via `@wordpress/core-data` in `src/app/hooks/use-followers.ts`; actor-profile fields (`actor_info`, webfinger, avatar…) come from that cached row.
* The proxy endpoint exists: `includes/rest/class-proxy-controller.php` — `POST /activitypub/1.0/proxy` (+ `/proxy/stream`), `permission_callback = verify_authentication` (OAuth). Advertised as `endpoints.proxyUrl` / `proxyEventStream` in `includes/model/class-user.php` and `class-blog.php`.

## Scope

* Add a proxy-backed actor resolver in the C2S client/binding: given an actor IRI, `POST /proxy` with the id and feed the returned actor object into the read model (per Automattic/wordpress-activitypub#3427).
* Route the app's actor-object reads through it, replacing the direct `ap_actor` core-data read for profile data.

## Open question (confirm scope)

* The follower/following **relationship** (who follows you) is inherently *local* — only the actor **object/profile** resolution should move to `proxyUrl`; the membership/list source likely stays `ap_actor`. Confirm whether this issue covers only profile resolution or also the list source.

## Auth

* `/proxy` is OAuth-authenticated (`verify_authentication`), so it shares the admin-token concern tracked in Automattic/wordpress-activitypub#3428.

Relates to Automattic/wordpress-activitypub#3427 (client core) / Automattic/wordpress-activitypub#3428 (binding).