Add Svelte error-boundary guards for LexBox page throws

[hahn-kev-bot]

we now catch render errors in the UI so users don't get stuck without knowing what's going on.

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AI summary

LexBox SvelteKit now wraps layout children in Svelte 5 <svelte:boundary> so a page render throw can recover in place instead of taking down the whole tree.

• New ErrorBoundary traces with app.error.source: svelte-boundary and never resets from onerror
• ErrorRecovery shows errors.apology, the error message, Retry, and Go home (rel="external" for a full document load)
• Root layout wraps only {@render children?.()} (footer, overlay, UnexpectedErrorAlert stay outside)
• Authenticated layout wraps children inside <Layout> so AppBar survives
• Sandbox has a one-shot Throw render error control to prove Retry remounts sandbox ok
• errors.retry added in en/es/fr/ko

+error.svelte and UnexpectedErrorAlert are unchanged.

Test plan

• pnpm check in frontend/ (0 errors)
• Open /sandbox, click Throw render error
  • Other cards stay visible
  • Fallback shows apology, sandbox render error, Retry, and Go home
  • Retry restores sandbox ok
  • Go home does a full navigation (rel="external"); unauthenticated users land on /login
• Window-level errors still use UnexpectedErrorAlert (unchanged)

[coderabbitai]

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[rmunn]

LGTM