fix: use artist DID as signer for atprotofans validation (#635)

direct atprotofans contributions use the artist's DID as the signer,
not the broker DID. the broker DID is only used when plyr.fm is a
registered platform that created the support template.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/src/routes/u/[handle]/+page.svelte

···
 	import { APP_NAME, APP_CANONICAL_URL } from '$lib/branding';
 	import type { PageData } from './$types';
 
-	// atprotofans broker DID for validateSupporter calls
-	const ATPROTOFANS_BROKER_DID = 'did:plc:7ewx3bksukdk6a4vycoykhhw';
 
 	// receive server-loaded data
 	let { data }: { data: PageData } = $props();
···
 			const url = new URL('https://atprotofans.com/xrpc/com.atprotofans.validateSupporter');
 			url.searchParams.set('supporter', auth.user.did);
 			url.searchParams.set('subject', artist.did);
-			url.searchParams.set('signer', ATPROTOFANS_BROKER_DID);
+			url.searchParams.set('signer', artist.did);
 
 			const response = await fetch(url.toString());
 			if (response.ok) {