Dedupe useErrorClassification across consumers with createSharedComposable

## Problem

`useErrorClassification()` builds an independent `computed` per caller, so simultaneously mounted consumers (RightSidePanel via `useHasBlockingError`, ErrorGroupList via `useErrorGroups`, the error overlay state) each re-run `classifyPanelErrors` — the full absorption matching over nodeErrors × missingModels × missingMedia — on every store change. Pure inputs mean no drift, only duplicated work, but the composable name implies shared derived state while delivering per-caller instances.

## Suggested fix

Wrap with `createSharedComposable` from `@vueuse/core` so the computed is deduped across consumers and disposed when the last one unmounts. One-line change plus a test that two consumers observe the same instance.

Found during the multi-agent review of #14440.

Start by locating the useErrorClassification composable and its existing tests, then inspect how useHasBlockingError, useErrorGroups, and the error overlay consume it. Wrap the composable with createSharedComposable and add a test showing two consumers observe the same instance; done means the shared state is deduplicated and is disposed after the last consumer unmounts.

Written by the indexing model from the issue text.