feat(call): screenshare over the widget API with Capability Delegation (#43)

The call bar's Start/Stop Screenshare no longer clicks EC's hidden button
on Chromium (incl. WebView2): it sends io.lotus.set_screenshare with
postMessage `{ delegate: "display-capture" }` inside the user's click, so
the frame can call getDisplayMedia on engines that require the click.
matrix-widget-api has no postMessage options, so its sendInternal is
swapped for that one synchronous send; delegation needs the frame's real
origin, not `*`.

Firefox, Safari and WebKitGTK (Linux desktop) have no delegation and
still click EC's button (needs same-origin, which is still on). Gated on
the fork reporting `screenshareAction` in controls_state.

Pins @lotusguild/element-call-embedded 0.25.0-lotus.17.

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

package-lock.json

@@ -81,7 +81,7 @@
       },
       "devDependencies": {
         "@axe-core/playwright": "4.13.0",
-        "@lotusguild/element-call-embedded": "0.25.0-lotus.16",
+        "@lotusguild/element-call-embedded": "0.25.0-lotus.17",
         "@playwright/test": "1.63.0",
         "@rollup/plugin-inject": "5.0.5",
         "@rollup/plugin-wasm": "6.2.2",
@@ -2695,9 +2695,9 @@
       "integrity": "sha512-dfLbk+PwWvFzSxwk3n5ySL0hfBog779o8h68wK/7/APo/7cgyWp5jcXockbxdk5kFRkbeXWm4Fbi9FrdN381sA=="
     },
     "node_modules/@lotusguild/element-call-embedded": {
-      "version": "0.25.0-lotus.16",
-      "resolved": "https://code.lotusguild.org/api/packages/LotusGuild/npm/%40lotusguild%2Felement-call-embedded/-/0.25.0-lotus.16/element-call-embedded-0.25.0-lotus.16.tgz",
-      "integrity": "sha512-X/MzZ2Ip7IpZD3B+SkBoBzgMfURyZuYD6OaGL9r7sw2nl7Y/0JzgpT8RWV12lu4DlOfWbbeO+ka1l+pTYVewdQ==",
+      "version": "0.25.0-lotus.17",
+      "resolved": "https://code.lotusguild.org/api/packages/LotusGuild/npm/%40lotusguild%2Felement-call-embedded/-/0.25.0-lotus.17/element-call-embedded-0.25.0-lotus.17.tgz",
+      "integrity": "sha512-dxzo2Nw1NEEOe+twQXITAZtb+X7fFJdtqnElsFOu0ArccTB1QLbibc3F0XFp+BTfT5PZIyNaIp7fDArVSnj6qg==",
       "dev": true
     },
     "node_modules/@matrix-org/matrix-sdk-crypto-wasm": {