Move the macOS `FullscreenController` (NSViewRepresentable that drives native
fullscreen) into its own FullscreenController.swift, and `ApprovalRequest` (the
pending-trust-decision value type) into ApprovalRequest.swift, out of the
1041-line ContentView.swift. Both were file-`private`; dropped to internal so
ContentView (same module) still references them across files. StreamView*.swift
and the connection body are untouched. Pure move; no behavior change.
Verified: `swift build` (macOS) — Build complete.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
