Refactor/dark light toggle error boundary

[MerlinTheWhiz]

Closes #537

Changes made:

• Added component-level ErrorBoundary protection around ThemeToggle in src/components/ui/theme-toggle.tsx:70.
• Added an accessible disabled fallback button for toggle failures in src/components/ui/theme-toggle.tsx:9.
• Preserved existing toggle behavior and added type="button" plus decorative icon aria-hidden attributes.
• Added focused tests in src/components/ui/tests/theme-toggle.test.tsx:27 and src/components/ui/tests/theme-toggle.test.tsx:43.

Validation:

• Passed: pnpm vitest run src/components/ui/tests/theme-toggle.test.tsx

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