Summary

• Show a warning notice when desktop reasoning effort changes are rejected instead of silently refreshing back to auto.
• Classify common non-fatal blockers: active work, session lease conflicts, startup-in-progress, startup failure, and stale tab retries.
• Add focused frontend coverage for the notice mapping and rejected effort switch path.

Why

Refs #5956. The effort menu can expose max correctly while the controller rebuild fails; the UI previously swallowed that error, making the click look like it did nothing.

The root session lease race is already fixed by #5968. This PR is the UX follow-up: it surfaces rejected effort changes with actionable frontend notices so future backend blockers are visible instead of looking like a no-op.

Validation

• pnpm --dir desktop/frontend exec tsx src/__tests__/use-controller-meta.test.ts
• pnpm --dir desktop/frontend exec tsx src/__tests__/use-controller-cancel-reconcile.test.tsx
• pnpm --dir desktop/frontend exec tsc --noEmit -p tsconfig.test.json
• git diff --check