[Fix]: pre-existing TS2322 in src/mdx-components.tsx + lint warnings

[abdout]

Description

When CI ran pnpm typecheck for the first time (in PR #10), it surfaced a TypeScript error that's been hiding in the repo:

```
src/mdx-components.tsx(316,3): error TS2322: Type '{ h1: ...; h2: ...; ... 53 more ...; SpellStats: () => JSX.Element; }' is not assignable to type 'Record<string, ComponentType>'.
```

`pnpm lint` also exits 1 (next-lint findings — diff TBD).

PR #10 marks the typecheck and lint jobs as `continue-on-error: true` so they report but don't block the merge. This issue tracks fixing them properly so we can flip the gate back on.

Steps to reproduce

```bash
pnpm install
pnpm typecheck # exits 2
pnpm lint # exits 1
```

Acceptance

• Typecheck passes (`pnpm typecheck` exits 0)
• Lint passes (`pnpm lint` exits 0)
• Flip continue-on-error back to false in `.github/workflows/pr-check.yml` typecheck + lint jobs

Refs

Refs #9 (the unified workflow PR that surfaced this)