feat(call): optional Element Call origin via config.json elementCallUrl (#43)

Groundwork for serving the call page from its own origin
(call.chat.lotusguild.org). Inert until config.json sets `elementCallUrl`:
without it the bundled same-origin page is used exactly as today.

- callPageUrl: resolves `elementCallUrl` — absolute https only (http only on
  localhost for development); anything else, and the desktop app, fall back
  to the bundled page so a bad value can't break calls. Set once from the
  loaded client config.
- CallEmbed builds the widget URL from it; the widget origin (used by the
  message guard and Capability Delegation) follows automatically.
- Soundboard: a host blob: URL can't be fetched from another origin, so
  io.lotus.inject_audio now also carries the clip's bytes (`audio`). Forks
  that predate it ignore the field and use `url`, so this is safe on the
  released fork.

Needs element-call's lotus-call-origin branch (host-origin message check +
inject_audio bytes) released and pinned before `elementCallUrl` is set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/plugins/call/CallEmbed.ts

@@ -29,6 +29,7 @@ import { CallControl } from './CallControl';
 import { CallControlState } from './CallControlState';
 import { verifyDenoiseAssets } from './denoiseSmokeCheck';
 import { canDelegateCapability } from './utils';
+import { getCallPageUrl } from './callPageUrl';
 import { restrictWidgetMessages } from '../widgetTransport';
 
 // Maximum time to wait for the embedded Element Call iframe to progress from
@@ -254,10 +255,14 @@ export class CallEmbed {
       params.append('sendNotificationType', CallEmbed.dmCall(intent) ? 'ring' : 'notification');
     }
 
-    const widgetUrl = new URL(
-      `${trimTrailingSlash(import.meta.env.BASE_URL)}/public/element-call/index.html`,
-      window.location.origin,
-    );
+    // [Gitea #43] On its own origin when config.json sets elementCallUrl.
+    const externalPage = getCallPageUrl();
+    const widgetUrl = externalPage
+      ? new URL(externalPage)
+      : new URL(
+          `${trimTrailingSlash(import.meta.env.BASE_URL)}/public/element-call/index.html`,
+          window.location.origin,
+        );
     widgetUrl.search = params.toString();
 
     const options: IWidget = {