feat(apple/input): desktop (absolute) mouse mode on macOS — remote-desktop sweep M1

Folds the parked client-side-cursor machinery (cursorMode auto/always/
never, hidden while disabled) into the cross-client mouse model:
MouseInputMode capture|desktop under DefaultsKey.mouseMode, picked in
Settings ▸ Keyboard & mouse (macOS), resolved at session start and
gated off on gamescope hosts (relative-only EIS).

Desktop model = the un-neutered absolute path with the SDL cursor
policy: pointer never disassociated (enters/leaves the stream freely),
monitor forwards letterboxed absolute positions, and the local cursor
hides only while over the view via an invisible-cursor rect (the
host's composited cursor is the one you see; AppKit manages the rect,
so no hide/unhide balancing). ⌘⇧C becomes ⌃⌥⇧M — the same chord as
the SDL clients — flipping the model live with an atomic
release/re-engage.

Verified: swift build + full test suites green (macOS arm64).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -89,6 +89,7 @@ struct SettingsView: View {
     @State var customMode = false
     #endif
     #if os(macOS)
+    @AppStorage(DefaultsKey.mouseMode) var mouseMode = MouseInputMode.capture.rawValue
     @AppStorage(DefaultsKey.speakerUID) var speakerUID = ""
     @AppStorage(DefaultsKey.micUID) var micUID = ""
     @AppStorage(DefaultsKey.micChannel) var micChannel = 0