Fix Graphical in Challenges

[twells46]

Fixes #586 by adding a fallback to default code when current code is empty (as it is in Graphical). Also enabled Graphical compilation by adding case to ChallengeRoot onRunClick_.

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Note

Improves challenge execution flow and adds graphical language support.

• Adds graphical case in onRunClick_ to start the worker with language: 'graphical'
• Falls back to ProgrammingLanguage.DEFAULT_CODE[language] when saved code is undefined for run, download, AI tutor requests, and editor rendering
• Updates loading guard to check code === undefined (allowing empty strings)
• Includes graphical in ChallengeCompletion.EMPTY.code

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