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/features/call/CallSoundboard.tsx

@@ -115,7 +115,12 @@ export function CallSoundboard({ callEmbed }: CallSoundboardProps) {
       try {
         const url = await resolveClipObjectUrl(mx, flat.clip.url);
         const vol = (flat.clip.volume / 100) * master;
-        const result = await callEmbed.control.injectAudio(url, vol);
+        // [Gitea #43] Send the bytes too: the call page may be on another
+        // origin, where this blob: URL can't be fetched.
+        const clipBytes = await fetch(url)
+          .then((r) => r.arrayBuffer())
+          .catch(() => undefined);
+        const result = await callEmbed.control.injectAudio(url, vol, clipBytes);
         if (!result.played) {
           // [EC#13] Refused fork-side (only reason today: local mic muted) —
           // don't play it locally either, or the user would think it went out.