Add difficulty + platform filters to Auth Methods Testing page (post-MVP)

Inline code marker.

**Location:** `src/pages/AuthMethodsTestingPage.tsx:68`

```
{/* TODO: difficulty + platform filters (post-MVP). */}
```

The marker stays in the code (annotated `(#this)` in place).

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_Migrated from `src/pages/AuthMethodsTestingPage.tsx:68`._