feat(clients/apple): user-configurable Allow VRR (macOS + iOS)

Add an "Allow VRR" toggle (Settings → Display; default on, non-tvOS) that
hands the presenter's display link a wide frame-rate RANGE — preferred =
the stream rate — so a ProMotion / adaptive-sync display can vary its
physical refresh to match the stream. `SessionPresenter.syncFrameRate`
now runs on macOS too: on it requests min 24 / max display / preferred
stream Hz; off restores the prior behavior (macOS free-runs at the native
rate, iOS keeps its 30 Hz floor). A no-op on fixed-refresh displays.

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

clients/apple/Sources/PunktfunkClient/Settings/SettingsView.swift

@@ -28,6 +28,9 @@ struct SettingsView: View {
     #if os(macOS)
     @AppStorage(DefaultsKey.vsync) var vsync = false
     #endif
+    #if !os(tvOS)
+    @AppStorage(DefaultsKey.allowVRR) var allowVRR = true
+    #endif
     @AppStorage(DefaultsKey.hdrEnabled) var hdrEnabled = true
     @AppStorage(DefaultsKey.enable444) var enable444 = true
     @AppStorage(DefaultsKey.libraryEnabled) var libraryEnabled = false
@@ -109,6 +112,7 @@ struct SettingsView: View {
             Form {
                 presenterSection
                 hdrSection
+                vrrSection
                 vsyncSection
                 windowSection
                 statisticsSection
@@ -238,6 +242,7 @@ struct SettingsView: View {
             Form {
                 presenterSection
                 hdrSection
+                vrrSection
                 statisticsSection
             }
             .formStyle(.grouped)