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/PunktfunkShared/DefaultsKeys.swift

@@ -94,8 +94,11 @@ public enum DefaultsKey {
     /// stays 4:2:0). Sharper text/UI at the cost of more bandwidth.
     public static let enable444 = "punktfunk.enable444"
     public static let hosts = "punktfunk.hosts"
-    /// Client-side cursor mode: "auto" (shown only in gamescope sessions), "always", "never".
-    public static let cursorMode = "punktfunk.cursorMode"
+    /// Physical-mouse model (macOS): "capture" (pointer lock + relative, the default) or
+    /// "desktop" (uncaptured absolute pointer) — the cross-client `mouse_mode`. Replaces the
+    /// never-shipped "punktfunk.cursorMode" (auto/always/never client-side-cursor setting,
+    /// which was hidden while disabled and had no readers).
+    public static let mouseMode = "punktfunk.mouseMode"
     /// Invert the scroll-wheel / two-finger-scroll direction sent to the host (both axes). Off by
     /// default: the local (natural-scrolling) sign passes through untouched. When on, the sign is
     /// negated at the single scroll sink (`InputCapture.sendScroll`), so it flips consistently across