Video feed was captured while user has disabled video

A user participating in a video call toggles their camera off via the UI, which sets the stream's `videoState` to `.stopped`. Because the `SelfVideoPreviewView` did not react to this state change, the underlying `AVSVideoPreview` continued capturing video. The video stream therefore remained active and was transmitted to other call participants, contrary to the user's expectation that their camera was disabled. No special network position or authentication bypass is required — any user in a video call can trigger the bug by simply disabling their camera.

The vulnerability resides in `SelfVideoPreviewView` in the Wire iOS client. The class previously did not update the video capture state when the `stream` property changed, meaning that disabling the camera (setting `videoState` to `.stopped`) did not stop the underlying `AVSVideoPreview` capture. The patch adds a `didSet` observer on `stream` that calls `updateCaptureState()`, which stops capture when the video state is not `.started`.

The fix adds a `didSet` observer on the `stream` property of `SelfVideoPreviewView` that calls a new `updateCaptureState()` method. That method checks whether `stream.videoState == .started`; if so it calls `startCapture()`, otherwise it calls `stopCapture()`. Additionally, in `VideoGridViewController.updateSelfPreview()`, when the self stream is not found, the code now explicitly calls `stopCapture()` on the preview view. These changes ensure that the video capture is always stopped when the user disables their camera, closing the privacy leak.