Fixed linked list detection bug and guard visualization errors

[faenwitee]

Proposed Changes

Fixed #332. This PR prevents Linked List View crashes by handling legacy saved attribute formats and missing labels safely. The toggle remains available for all questions, with “No linked lists detected” displayed when no supported structure is found. An error boundary contains visualization failures so the editor remains usable. The changes also remove unused code and extract shared reference helpers.

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Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)	X
♻️ Refactoring (internal change to codebase, without changing functionality)	X
🚦 Test update (change that only adds or modifies tests)
📚 Documentation update (change that only updates documentation)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

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