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/pages/App.tsx

@@ -36,6 +36,7 @@ import { applyCustomAccent, removeCustomAccent } from '../utils/accentColor';
 import { zIndices } from '../styles/zIndex';
 import { OIDC_CALLBACK_PATH } from './paths';
 import { OidcCallback } from './auth/oidc/OidcCallback';
+import { resolveCallPageUrl, setCallPageUrl } from '../plugins/call/callPageUrl';
 
 // The emoji families (Twemoji when "Twitter emoji" is on, Twemoji flags on
 // Windows — see SystemEmojiFeature) must sit before the generic family, or the
@@ -220,23 +221,27 @@ function App() {
                     <ConfigConfigError error={err} retry={retry} ignore={ignore} />
                   )}
                 >
-                  {(clientConfig) => (
-                    <ClientConfigProvider value={clientConfig}>
-                      <QueryClientProvider client={queryClient}>
-                        <JotaiProvider>
-                          <AppearanceEffects />
-                          <TauriEffects />
-                          <DesktopChrome>
-                            <RouterProvider router={createRouter(clientConfig, screenSize)} />
-                          </DesktopChrome>
-                          <SeasonalEffect />
-                          <NightLightOverlay />
-                          <LotusToastContainer />
-                        </JotaiProvider>
-                        <ReactQueryDevtools initialIsOpen={false} />
-                      </QueryClientProvider>
-                    </ClientConfigProvider>
-                  )}
+                  {(clientConfig) => {
+                    // [Gitea #43] Idempotent: where the call page is loaded from.
+                    setCallPageUrl(resolveCallPageUrl(clientConfig.elementCallUrl, isTauri()));
+                    return (
+                      <ClientConfigProvider value={clientConfig}>
+                        <QueryClientProvider client={queryClient}>
+                          <JotaiProvider>
+                            <AppearanceEffects />
+                            <TauriEffects />
+                            <DesktopChrome>
+                              <RouterProvider router={createRouter(clientConfig, screenSize)} />
+                            </DesktopChrome>
+                            <SeasonalEffect />
+                            <NightLightOverlay />
+                            <LotusToastContainer />
+                          </JotaiProvider>
+                          <ReactQueryDevtools initialIsOpen={false} />
+                        </QueryClientProvider>
+                      </ClientConfigProvider>
+                    );
+                  }}
                 </ClientConfigLoader>
               </FeatureCheck>
             </ScreenSizeProvider>