feat(clients/apple): cross-client shortcuts + start banner, opt-in V-Sync, presenter rework

Align the macOS/iPad Stream menu on the cross-client Ctrl+Alt+Shift set
the Windows and Linux clients reserve — Release Mouse (⌃⌥⇧Q), Disconnect
(⌃⌥⇧D), HUD toggle (⌃⌥⇧S) — with ⌘⎋ kept as the macOS/iPad capture
toggle, and surface them on a 6-second banner at stream start.

Add an opt-in V-Sync present mode (punktfunk.vsync, default OFF =
lowest-latency immediate present; PUNKTFUNK_PRESENT_MODE overrides for
A/B), with the presenter reworked to a frame-arrival-triggered render
thread across Stage2Pipeline / MetalVideoPresenter / SessionPresenter,
plus the windowed title-bar safe-area handling.

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

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

@@ -306,6 +306,25 @@ extension SettingsView {
         #endif
     }
 
+    // macOS-only: iOS/tvOS layers always present on the display's vsync, so the choice only
+    // exists on the Mac (where the layer's own sync must stay off — see MetalVideoPresenter).
+    @ViewBuilder var vsyncSection: some View {
+        #if os(macOS)
+        Section {
+            Toggle("V-Sync", isOn: $vsync)
+        } header: {
+            Text("Presentation")
+        } footer: {
+            Text("Off (default): each frame is shown as soon as it's ready — lowest latency, "
+                + "but frame timing can look uneven and fullscreen may tear. On: frames flip "
+                + "in step with the display's refresh — evenly paced, up to one refresh of "
+                + "added latency. Applies from the next session.")
+                .font(.geist(12, relativeTo: .caption))
+                .foregroundStyle(.secondary)
+        }
+        #endif
+    }
+
     // Stage-2 (Metal/VTDecompressionSession) is the default and only user-visible presenter — it
     // recovers from a wedged decoder, where stage-1's AVSampleBufferDisplayLayer freezes hard on a
     // lost HEVC reference. Stage-1 is kept reachable as a DEBUG-only override for diagnostics, like

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

@@ -25,6 +25,9 @@ struct SettingsView: View {
     @AppStorage(DefaultsKey.gamepadType) var gamepadType = 0
     @AppStorage(DefaultsKey.bitrateKbps) var bitrateKbps = 0
     @AppStorage(DefaultsKey.presenter) var presenter = "stage2"
+    #if os(macOS)
+    @AppStorage(DefaultsKey.vsync) var vsync = false
+    #endif
     @AppStorage(DefaultsKey.hdrEnabled) var hdrEnabled = true
     @AppStorage(DefaultsKey.enable444) var enable444 = true
     @AppStorage(DefaultsKey.libraryEnabled) var libraryEnabled = false
@@ -106,6 +109,7 @@ struct SettingsView: View {
             Form {
                 presenterSection
                 hdrSection
+                vsyncSection
                 windowSection
                 statisticsSection
             }