Use standard pdfjs-dist entry point instead of legacy subpath

## Problem

Lector internally imports from `pdfjs-dist/legacy/build/pdf.mjs`:

```js
// src/lib/pdfjs.ts
pdfJsPromise = import('pdfjs-dist/legacy/build/pdf.mjs');
```

This is a different module instance than what `import('pdfjs-dist')` returns. In pdfjs-dist v5, each build entry has its own `GlobalWorkerOptions` class with private static fields, so configuring the worker on the standard entry point has no effect on lector's internal instance:

```ts
import { GlobalWorkerOptions } from "pdfjs-dist";
GlobalWorkerOptions.workerSrc = new URL(
"pdfjs-dist/build/pdf.worker.mjs",
import.meta.url
).toString();
// ^ This does NOT affect lector — it uses a separate GlobalWorkerOptions
```

This causes `No "GlobalWorkerOptions.workerSrc" specified.` at runtime (related: #17).

## Suggested fix

In `src/lib/pdfjs.ts`, change the import to the standard entry point:

```diff
- pdfJsPromise = import('pdfjs-dist/legacy/build/pdf.mjs');
+ pdfJsPromise = import('pdfjs-dist');
```

This way consumers configure one `GlobalWorkerOptions` and lector sees it. Projects that need the legacy build can still alias `pdfjs-dist` → `pdfjs-dist/legacy/build/pdf.mjs` in their bundler config — but the default should use the package's declared main entry.

## Workaround

For anyone hitting this today, add a Vite resolve alias:

```ts
resolve: {
alias: {
"pdfjs-dist/legacy/build/pdf.mjs": "pdfjs-dist/build/pdf.mjs",
},
}
```