chore(release): drop flatpak, add macOS .dmg, hide Windows console

- .github/workflows/release.yml: remove the build-flatpak job; matrix-
  add macOS (arm64 + x86_64) that builds H7CAD.app by hand, renders
  .icns from assets/logo.svg via librsvg + iconutil, and packages a
  UDZO .dmg. AppImage path updated to packaging/H7CAD.desktop.
- packaging/H7CAD.desktop: moved out of flatpak/ so the AppImage job
  doesn't depend on a Flatpak layout.
- packaging/Info.plist: macOS bundle template; __VERSION__ swapped
  for the release tag at build time.
- flatpak/: deleted (manifests, metainfo, desktop, builder cache).
- src/main.rs: `#![cfg_attr(all(windows, not(debug_assertions)),
  windows_subsystem = "windows")]` so the Windows GUI binary no
  longer spawns a console window alongside it (debug builds keep
  the console for log output).
- README: add macOS install section, document Gatekeeper / SmartScreen
  on first launch since the binaries are unsigned.

Bump to 0.3.5.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packaging/H7CAD.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Name=H7CAD
+Comment=A CAD application for 2D/3D drawing and design
+Exec=H7CAD
+Icon=io.github.HakanSeven12.H7CAD
+Terminal=false
+Type=Application
+Categories=Graphics;Engineering;
+MimeType=image/vnd.dxf;
+Keywords=CAD;DXF;drawing;design;engineering;2D;3D;
+StartupWMClass=H7CAD