refactor(apple/W8): extract FullscreenController + ApprovalRequest from ContentView

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>

clients/apple/Sources/PunktfunkClient/ApprovalRequest.swift

@@ -0,0 +1,10 @@
+import Foundation
+import PunktfunkKit
+
+/// A fresh `pair=required`/unknown host pending a trust decision: drives both the "request access
+/// vs. pair with PIN" choice and the subsequent approval wait. `advertisedFingerprint` is the
+/// discovered host's advertised cert (nil for a manually-typed host → trust-on-first-use).
+struct ApprovalRequest {
+    let host: StoredHost
+    let advertisedFingerprint: Data?
+}