docs: expand README with build instructions, tech stack, and test info

[JWhite212]

Summary

• Replaced the one-sentence README ("The implementation of a given JavaFX project…") with a proper project description.
• Removed the dismissive "a given JavaFX project" phrasing; describes the calculator professionally as a JavaFX 17 + Maven project.
• Added a build/run section with both mvn and Maven Wrapper commands for macOS, Linux, and Windows.
• Added a tests section with the mvn test command.
• Added a tech stack table (Java 17, JavaFX 17.0.2, JUnit Jupiter 5.8.2, Maven) derived from pom.xml.
• Retained the existing screenshot.

Test plan

• Confirm mvn clean javafx:run launches the calculator UI.
• Confirm mvn test runs the JUnit 5 suite cleanly.
• Verify tech stack versions match pom.xml.

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