Fix build: TileVideoView composes VideoView (can't subclass a non-open class)

Build error: `cannot inherit from non-open class 'VideoView' outside of its
defining module` — VideoView is public, not open, so it can't be subclassed
in the plugin module. Reworked TileVideoView from a VideoView subclass into a
UIView CONTAINER that holds a VideoView: the container is frame-synced to the
web tile rect, the pinch/pan zoom transform lives on the inner video (so it
never fights the position poll), and the name/mute overlays sit on the
container so they no longer scale with zoom. track/layoutMode are forwarded.

Also silenced the extension warning: SampleHandler now restates
`@unchecked Sendable`.

The broadcast extension itself compiled + linked in the failed build, so the
SPM-linked extension injection + App Group setup are working.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mobile/ios-broadcast/SampleHandler.swift

@@ -13,4 +13,4 @@ import LiveKit
 // convention, keyed off this extension's bundle id (com.bizgaze.connect.broadcast) and the default app group
 // group.<appBundleId>. Both are also set explicitly on the app side (RTCScreenSharingExtension /
 // RTCAppGroupIdentifier in Info.plist) so there's no ambiguity.
-class SampleHandler: LKSampleHandler {}
+final class SampleHandler: LKSampleHandler, @unchecked Sendable {}