Summary

• Localize desktop model/token-mode switch failures, backend notices, and selected frontend throw paths.
• Map settings/provider and hooks JSON validation errors through the active desktop language.
• Add regression coverage for switch failure and backend notice localization.

Verification

• pnpm exec tsc --noEmit
• pnpm run test:typecheck
• pnpm exec tsx src/tests/use-controller-meta.test.ts
• pnpm exec tsx src/tests/settings-refresh-snapshot.test.tsx
• git diff --check

Cache impact

Cache-impact: none, desktop frontend localization and settings UI error handling only; no provider-visible prompt/tool/cache surfaces changed.
Cache-guard: pnpm exec tsc --noEmit; pnpm run test:typecheck; pnpm exec tsx src/tests/use-controller-meta.test.ts; pnpm exec tsx src/tests/settings-refresh-snapshot.test.tsx
System-prompt-review: N/A