feat(desktop): call page on its own loopback origin, opt-in (#43)

The desktop app loads the bundled Element Call page from its own origin
(http://localhost:<port>), so the call frame can read the app's storage
(login token) and DOM — the hole #43 closed on the web by moving the page
to call.chat.lotusguild.org.

The desktop's local server can also answer on http://127.0.0.1:<port>: the
same server and bundle, but a different origin (and still a secure
context). resolveDesktopCallPageUrl loads the bundled page from there when
the desktop config sets `desktopCallOrigin`:
- only a loopback http origin on the SAME port as the app, no path, query
  or credentials;
- only when the app itself runs on http://localhost (release builds; debug
  builds on tauri:// keep the same-origin page);
- unset (every desktop build until cinny-desktop opts in, together with the
  server bind, CSP and permission changes it needs): unchanged.

The web app is unchanged (elementCallUrl as before).

Tested in a simulated desktop app (Tauri bridge stub + the desktop
config.json, served on localhost and 127.0.0.1) against a local Synapse +
LiveKit, two users: call page from http://127.0.0.1:<port>, parentUrl =
the app origin; the frame gets SecurityError on parent.localStorage and
parent.document (same-origin control: readable); join, speaking indicator,
mic off/on, screenshare start/stop, layout switch and hang-up all work, no
page errors — 12/12 in 5 of 6 runs, like the same-origin control (3 of 4;
the misses on both sides were the local LiveKit connection).

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

src/app/pages/App.tsx

@@ -36,7 +36,11 @@ 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';
+import {
+  resolveCallPageUrl,
+  resolveDesktopCallPageUrl,
+  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
@@ -223,7 +227,15 @@ function App() {
                 >
                   {(clientConfig) => {
                     // [Gitea #43] Idempotent: where the call page is loaded from.
-                    setCallPageUrl(resolveCallPageUrl(clientConfig.elementCallUrl, isTauri()));
+                    setCallPageUrl(
+                      isTauri()
+                        ? resolveDesktopCallPageUrl(
+                            clientConfig.desktopCallOrigin,
+                            window.location.origin,
+                            import.meta.env.BASE_URL,
+                          )
+                        : resolveCallPageUrl(clientConfig.elementCallUrl, false),
+                    );
                     return (
                       <ClientConfigProvider value={clientConfig}>
                         <QueryClientProvider client={queryClient}>