feat(lotus): in-frame screenshare button where the host can't delegate (cinny #43)

Firefox, Safari and WebKitGTK have no Capability Delegation, so the host
can't start a share from its call bar: getDisplayMedia needs the click in
this frame. Until now the host clicked EC's hidden footer button through
the DOM, which only works while the frame is same-origin.

With lotusHostControls + lotusFrameScreenshare the fork now shows EC's own
screenshare button (floating bottom-right, footer still hidden), so the
click is native. controls_state reports frameScreenshare so the host hides
its own button. New toWidget io.lotus.set_frame_screenshare { visible }
lets the host hide it while the room's call policy forbids screensharing
(never while a share is live, so it can still be stopped).

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

src/index.css

@@ -91,7 +91,7 @@ body.lotus-transparent[data-background="gradient"]::before {
 }
 
 /* [cinny #43] lotusHostControls=1: the host renders its own call bar, so EC's
-footer is hidden but stays in the DOM and in layout-independent position. */
+footer is hidden (kept in the DOM but out of the layout flow). */
 body.lotus-host-controls [data-testid="footer-container"] {
   position: absolute !important;
   visibility: hidden !important;