Switch viewer close to SharedFlow command bus

[bartwell]

Motivation

• Replace platform-specific close managers and holders with a single cross-platform command bus so the UI layers handle closing consistently.
• Preserve the public API Kick.close() as the programmatic entry point for requesting viewer close.

Description

• Added a shared command bus ViewerCommands in main-runtime that exposes a closeRequests: SharedFlow<Unit> and requestClose() to emit close commands.
• Made KickImpl.close() emit a close request via ViewerCommands.requestClose() and kept the Kick public API and companion helper Kick.close() intact.
• Removed platform CloseManager implementations and holder utilities and wired UI layers to collect ViewerCommands.closeRequests: Android KickActivity now lifecycle-collects and calls finish(), JVM LaunchManager uses LaunchedEffect to window.dispose(), iOS IosSceneController collects in a MainScope job and calls dismiss(), and Wasm LaunchManager collects and removes the kick-viewer DOM element.
• Updated sample app to add a control panel close button (CONTROL_PANEL_CLOSE_BUTTON_ID) and to call Kick.close() and show an alert when the button is clicked.

Testing

• No automated tests were run as part of this change.

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Codex Task