Fix ErrorBoundary retry mechanism with key-based remounting

[Copilot]

React Error Boundaries don't automatically remount child trees after state reset. The "Try Again" button was resetting internal state but React continued rendering the error UI because the errored component tree remained mounted.

Changes

• ErrorBoundary component: Added resetKey state that increments on retry, wrapping children in <Fragment key={resetKey}> to force remount
• ErrorBoundary test: Corrected test sequence to update props before triggering retry (avoiding re-throw during remount)
• Type safety: Fixed componentStack null handling with nullish coalescing
• .gitignore: Excluded /assets/js/css build artifacts

Example

// Before: Children remained in errored state after handleRetry()
return children;

// After: Key change forces React to unmount/remount child tree
return <React.Fragment key={resetKey}>{children}</React.Fragment>;

This also improves production behavior—the retry button now actually recovers from errors instead of remaining stuck in error state.

Original prompt

test functionality and repair errors

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