Fry3DX 0.5: builds for Windows, Linux and Android, a phone layout, Apache 2.0

- Apache License 2.0
- Windows: cross-built with clang-cl (cmake --preset windows), a window
  program with its icon; Linux: a portable preset (SDL and the C++
  library inside); Android: android/build_apk.py makes the APK
- tools/package.py packs the downloads; a GitHub workflow builds all
  three and makes a release for a v* tag
- A phone layout (fry_touch, on by default on Android): the UI drawn
  bigger, panels when asked for, a bar of big buttons, one finger to
  turn the view, two to move and pinch, a long press for a right-click;
  files through the phone's file picker
- Settings and autosaves now in a Nexoniarz folder (copied over once)
- README rewritten, the website shows the downloads and the phone

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NMSrd5oe1GpUD7E6d825KP

engine/ui/ui.h

@@ -18,6 +18,12 @@ ImFont* font(Font f);
 // Sizes in pixels; scaled with the window height so 1080p looks like Source at 1080p.
 float scale();
 void beginFrame(float windowHeight);
+// The UI laid out `k` times smaller than the screen's pixels and drawn scaled up, everything
+// alike (a phone: its pixels are tiny). 1: as it is.
+void setPixelScale(float k);
+float pixelScale();
+// The backends call this between the platform's NewFrame and ImGui::NewFrame.
+void applyPixelScale();
 
 // The highlight colour: highlights, the logo.
 inline constexpr uint8_t kAccent[3] = {227, 34, 45};