feat: Xcode app project for the macOS client (Punktfunk.app)

clients/apple was a bare Swift package — fine for swift run/test, but app icons, a real
bundle (Info.plist, signing identity, TCC), and the normal Xcode build/run flow need an
app target. Punktfunk.xcodeproj (synchronized-folder format) wraps the SAME sources as
the CLI dev shell (Sources/PunktfunkClient) plus App/Assets.xcassets, and links
PunktfunkKit from the local package — no source duplication, both flows stay green:
swift build / swift test / swift run PunktfunkClient, and xcodebuild -scheme Punktfunk.

The asset catalog ships an empty AppIcon slot ready for the Icon Composer .icon
(drag in + set as App Icon + drop the placeholder; see README — including the actool
crash observed with the current icon bundle). Package tests on ⌘U need one GUI step
(Edit Scheme → Test → +); a hand-written package-test scheme reference doesn't resolve
headlessly.

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

clients/apple/App/Assets.xcassets/Contents.json

@@ -0,0 +1,6 @@
+{
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}