feat: validate API auth type field with enum instead of bare string

[rohilsurana]

Problem

In `src/types/config.ts`, the API auth schema uses a bare string for the `type` field:

```ts
const apiAuthSchema = z.object({
type: z.string(), // no validation
header: z.string(),
placeholder: z.string().optional(),
})
```

Typos like `apikey` instead of `apiKey`, or `bearer-token` instead of `bearer`, only fail at runtime when the playground tries to construct auth headers.

Suggested Fix

```ts
const apiAuthSchema = z.object({
type: z.enum(['apiKey', 'bearer', 'basic']),
header: z.string(),
placeholder: z.string().optional(),
})
```