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/Podfile.lock

@@ -0,0 +1,16 @@
+PODS:
+  - FlutterMacOS (1.0.0)
+
+DEPENDENCIES:
+  - FlutterMacOS (from `Flutter/ephemeral`)
+
+EXTERNAL SOURCES:
+  FlutterMacOS:
+    :path: Flutter/ephemeral
+
+SPEC CHECKSUMS:
+  FlutterMacOS: d0db08ddef1a9af05a5ec4b724367152bb0500b1
+
+PODFILE CHECKSUM: 54d867c82ac51cbd61b565781b9fada492027009
+
+COCOAPODS: 1.16.2