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/hooks/useClientConfig.ts

@@ -19,6 +19,12 @@ export type ClientConfig = {
 
   hashRouter?: HashRouterConfig;
   gifApiKey?: string;
+
+  /**
+   * [Gitea #43] Absolute https URL of the Element Call page on its own origin.
+   * Unset: the bundled copy on this origin. Ignored in the desktop app.
+   */
+  elementCallUrl?: string;
 };
 
 const ClientConfigContext = createContext<ClientConfig | null>(null);