Style Ruru error boundary to match GraphiQL theme

[benjaie]

Motivation

• The Ruru error fallback rendered plain, unstyled HTML after introducing an ErrorBoundary, which looks out of place inside the GraphiQL/Ruru UI.

Description

• Replace the plain fallback markup in grafast/ruru-components/src/ruru.tsx with structured GraphiQL-aligned markup using container classes (ruru-error-boundary, ruru-error-card, etc.) and preserved actions (Try again, destructive reset, cancel).
• Add comprehensive styles in grafast/ruru-components/ruru.css to match GraphiQL theming and variables, including card layout, header/lede, details panel, stack area, and button variants (-primary, -danger).
• Keep existing error handling behaviour intact (the random error title, tryAgain, destructive clear which clears localStorage and reloads, and the staged confirmation flow).

Testing

• No automated tests were run as part of this change (no unit/integration or CI runs performed).

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Codex Task