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/hooks/useClientConfig.ts

@@ -26,6 +26,15 @@ export type ClientConfig = {
    */
   elementCallUrl?: string;
 
+  /**
+   * [Gitea #43] Desktop only: the loopback origin the desktop app's local
+   * server also answers on (e.g. "http://127.0.0.1:44548"), to load the
+   * bundled call page from a different origin than the app
+   * ("http://localhost:44548"). Set by cinny-desktop together with the server
+   * and CSP changes it needs; unset keeps the same-origin call page.
+   */
+  desktopCallOrigin?: string;
+
   /**
    * Absolute https URL of the public web app (e.g. https://chat.lotusguild.org).
    * The desktop app sets it so it can hand calls it can't make to the browser.