add macOS desktop target with sandbox entitlements

OS-detecting Makefile (make run works on macOS/Linux/Windows), 1280x720
default window in MainMenu.xib, squared app icons, sandbox with scoped
SBPL exceptions for subprocess execution, _DARWIN_C_SOURCE for ptyc
compilation, expanded PATH merging Homebrew and ~/.local/bin for GUI
apps, native traffic dots skipped on macOS (titlebar owns them).

DebugProfile.entitlements is gitignored (machine-specific SBPL paths);
a template with __HOMEDIR__/__PROJECTS__/__SHELL__ placeholders is
committed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

macos/Flutter/Flutter-Debug.xcconfig

@@ -1 +1,2 @@
+#include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.debug.xcconfig"
 #include "ephemeral/Flutter-Generated.xcconfig"

macos/Flutter/Flutter-Release.xcconfig

@@ -1 +1,2 @@
+#include? "Pods/Target Support Files/Pods-Runner/Pods-Runner.release.xcconfig"
 #include "ephemeral/Flutter-Generated.xcconfig"