Remove legacy ErrorMessage.js and consolidate imports

## Problem

The legacy class-based `web/components/ErrorMessage.js` coexists with its modern TypeScript replacement at `web/components/messages/ErrorMessage.tsx`.

- **33 files** import from `components/ErrorMessage` (absolute path, resolves to the legacy `.js` file)
- **1 file** imports from `components/messages/ErrorMessage` (absolute path, the TS replacement)
- **5 files** use relative imports (`./ErrorMessage` or `./messages/ErrorMessage`)

Total: **39 files** referencing ErrorMessage across the codebase.

> **Note:** The equivalent `SuccessMessage.js` duplication was already resolved in PR #6873.

## Proposed Fix

**Option A (minimal diff):** Delete `ErrorMessage.js`, create `ErrorMessage.tsx` as a re-export shim:
```tsx
export { default } from 'components/messages/ErrorMessage'
```

**Option B (clean but large):** Delete `ErrorMessage.js`, update all 38 imports to `components/messages/ErrorMessage`.

The TypeScript `ErrorMessage` has the same props as the legacy version, so it is a drop-in replacement.

## Acceptance Criteria

- [ ] `web/components/ErrorMessage.js` is deleted
- [ ] All consumers resolve to the TypeScript `ErrorMessage.tsx`
- [ ] `npm run typecheck` passes
- [ ] No runtime errors when triggering error messages

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Part of the Design System Audit (#6606) · Epic: #6882