Add new Kotlin Multiplatform project to gui.

gui/README.md

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+This is a Kotlin Multiplatform project targeting Android, Desktop (JVM), built with the [Kotlin Toolchain](https://kotlin-toolchain.org/dev/).
+
+- [/androidApp](./androidApp) contains the Android application.
+- [/desktopApp](./desktopApp) contains the desktop (JVM) application.
+- [/shared](./shared) holds the code shared across your applications — Compose UI, business logic, and platform-specific implementations. [src](./shared/src) is for common code; the sibling `src@<platform>` folders (for example `src@android`) hold code compiled only for the platform named in the folder.
+
+The `kotlin` (macOS/Linux) and `kotlin.bat` (Windows) scripts in the project root are self-bootstrapping wrappers for the Kotlin Toolchain: they download the pinned toolchain version on first use, so no separate installation is required. Build the whole project with `./kotlin build`.
+
+### Running
+
+Use the run configurations provided by the run widget in your IDE's toolbar. You can also run each module from the command line:
+
+- Android app: `./kotlin run -m androidApp`
+- Desktop app: `./kotlin run -m desktopApp`
+
+### Testing
+
+Run the project's tests with `./kotlin test`, or `./kotlin test -m <module>` for a single module.
+
+---
+
+Learn more about [Kotlin Multiplatform](https://www.jetbrains.com/help/kotlin-multiplatform-dev/get-started.html), [Kotlin Toolchain](https://kotlin-toolchain.org/dev/), [Compose Multiplatform](https://kotlinlang.org/compose-multiplatform/).