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/plugins/call/callPageUrl.test.ts

@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { resolveCallPageUrl } from './callPageUrl';
+import { resolveCallPageUrl, resolveDesktopCallPageUrl } from './callPageUrl';
 
 const URL_OK = 'https://call.chat.example.org/public/element-call/index.html';
 
@@ -38,3 +38,44 @@ test('anything else falls back to the bundled page', () => {
     'data:text/html,x',
   ].forEach((v) => assert.equal(resolveCallPageUrl(v, false), undefined, String(v)));
 });
+
+const APP = 'http://localhost:44548';
+const PAGE = '/public/element-call/index.html';
+
+test('desktop: the bundled page from the loopback origin on the same port', () => {
+  assert.equal(
+    resolveDesktopCallPageUrl('http://127.0.0.1:44548', APP, '/'),
+    `http://127.0.0.1:44548${PAGE}`,
+  );
+  assert.equal(
+    resolveDesktopCallPageUrl('http://127.0.0.1:44548/', APP, '/app/'),
+    `http://127.0.0.1:44548/app${PAGE}`,
+  );
+});
+
+test('desktop: unset or anything but same-port loopback http keeps the same-origin page', () => {
+  [
+    undefined,
+    '',
+    'http://127.0.0.1:44549',
+    'http://127.0.0.1',
+    'https://127.0.0.1:44548',
+    'http://localhost:44548',
+    'http://[::1]:44548',
+    'http://10.0.0.5:44548',
+    'https://call.chat.lotusguild.org',
+    'http://127.0.0.1:44548/evil/',
+    'http://127.0.0.1:44548/?x=1',
+    'http://user:pw@127.0.0.1:44548',
+    'not a url',
+    42,
+  ].forEach((v) => assert.equal(resolveDesktopCallPageUrl(v, APP, '/'), undefined, String(v)));
+});
+
+test('desktop: only when the app itself runs on http://localhost (release builds)', () => {
+  const v = 'http://127.0.0.1:44548';
+  assert.equal(resolveDesktopCallPageUrl(v, 'tauri://localhost', '/'), undefined);
+  assert.equal(resolveDesktopCallPageUrl(v, 'http://tauri.localhost', '/'), undefined);
+  assert.equal(resolveDesktopCallPageUrl(v, 'https://chat.lotusguild.org', '/'), undefined);
+  assert.equal(resolveDesktopCallPageUrl(v, 'http://localhost', '/'), undefined);
+});