fix(mobile): keep iPhone compose visible and tappable#102

Carries upstream pingdotgg/t3code#24525 (ios-compose-button-layout, by PixPMusic) in the fork stack until it lands upstream. The commit is picked unchanged.

The iPhone compose button could lose its glass background, sit above the tab bar, and stop responding to taps. Its frame came only from UIKit tab controls found by accessibility identifier, and a missed lookup left it without a usable frame. It now measures the visible native tab controls without needing those identifiers, falls back to the bar's content area, keeps at least a 44 point target, and stays inside the host's right edge.

Drop this layer once the upstream PR merges and upstream-main moves past it.