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/lotus/lotusActions.ts

@@ -86,6 +86,12 @@ export enum LotusWidgetActions {
    * to report while focus is inside this frame (cinny #43). `[]` stops.
    */
   SetHotkeys = "io.lotus.set_hotkeys",
+  /**
+   * toWidget: `{ visible: boolean }` — whether the in-frame screenshare button
+   * (`lotusFrameScreenshare`) may show, e.g. false while the room's call
+   * policy forbids screensharing (cinny #43). Defaults to visible.
+   */
+  SetFrameScreenshare = "io.lotus.set_frame_screenshare",
   /**
    * fromWidget: a watched key went down/up here, or this window's focus
    * changed; see `LotusHotkeyReport` (cinny #43).
@@ -93,7 +99,7 @@ export enum LotusWidgetActions {
   Hotkey = "io.lotus.hotkey",
   /**
    * fromWidget: `{ screensharing: boolean, layout: "grid" | "spotlight" | null,
-   * screenshareAction: true, hotkeys: true }`
+   * screenshareAction: true, hotkeys: true, frameScreenshare: boolean }`
    * whenever either changes (cinny #43), so the host stops reading EC's DOM for
    * them. Its arrival also tells the host this fork supports the actions above.
    */
@@ -118,4 +124,5 @@ export const LOTUS_TO_WIDGET_ACTIONS: LotusWidgetActions[] = [
   LotusWidgetActions.ToggleReactions,
   LotusWidgetActions.SetScreenshare,
   LotusWidgetActions.SetHotkeys,
+  LotusWidgetActions.SetFrameScreenshare,
 ];