🧪 Testing: Increase UI component test coverage and resolve warnings

[saint2706]

🧪 Testing: Increased Coverage & Reliability

Discovery:
Identified uncovered UI components (CustomCursor, useTheme, EmptyStateIllustrations) dragging down overall branch coverage metrics and causing potential gaps in UI interactive logic validation. Also noted a React console/test warning about an invalid DOM property fetchpriority vs fetchPriority inside MarkdownRenderer.

Implementation:

1. Added full unit tests for CustomCursor testing fine-pointer simulation, requestAnimationFrame mocks for tracking coordinates, and hover interaction behaviors.
2. Verified useTheme correctly exports ThemeProvider contexts.
3. Added rendering tests for all EmptyStateIllustrations asserting proper className application and accurate useReducedMotion context overrides.
4. Corrected the lowercase fetchpriority DOM prop into the expected React fetchPriority camelCase to resolve test warnings.

Verification:
Test coverage threshold passed, build passes cleanly without unused variable TS6133 warnings, and pipeline should run safely.

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PR created automatically by Jules for task 8920499402242908342 started by @saint2706

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