Bug: createProlinkUrl additional params can overwrite the prolink payload

### Describe the bug

`createProlinkUrl()` can silently replace the encoded prolink payload when `additionalQueryParams` contains a `p` key.

The helper always sets the encoded payload under the reserved `p` query parameter first, then applies every extra query parameter with `URLSearchParams.set()`. Since `set()` overwrites existing keys, a caller that passes `{ p: "..." }` in `additionalQueryParams` gets a URL whose `p` value is no longer the `prolink` argument.

Affected file:

- `packages/account-sdk/src/interface/public-utilities/prolink/createProlinkUrl.ts`

Current code:

```ts
const link = new URL(url);
link.searchParams.set('p', prolink);
Object.entries(additionalQueryParams ?? {}).forEach(([key, value]) => {
link.searchParams.set(key, value);
});
```

### Steps

Minimal reproduction against the current implementation:

```ts
import { createProlinkUrl } from '@base-org/account/prolink';

const url = createProlinkUrl('real-prolink', 'https://base.app/base-pay', {
p: 'not-the-prolink',
ref: 'campaign',
});

console.log(new URL(url).searchParams.get('p'));
```

Observed result:

```text
not-the-prolink
```

The same can be expressed as a unit test beside `createProlinkUrl.test.ts`:

```ts
it('does not allow additional params to overwrite the prolink payload', () => {
const result = createProlinkUrl('real-prolink', undefined, { p: 'override' });
expect(new URL(result).searchParams.get('p')).toBe('real-prolink');
});
```

### Expected behavior

The encoded prolink payload should remain authoritative for the reserved `p` parameter. Additional query parameters should not be able to overwrite it silently.

Possible fixes:

- apply `additionalQueryParams` first, then set `p`, or
- reject `additionalQueryParams.p` with a clear error.

Either behavior would be safer than returning a URL where the payload no longer matches the `prolink` argument.

### Version

Current `master` at `79355e2445889c3ad8a2689d5759978dc9403151`.

### Additional info

I searched existing issues/PRs for `createProlinkUrl`, `additionalQueryParams p`, and prolink query parameter overwrite and did not find an existing report.

### Desktop

N/A

### Smartphone

N/A

Start with packages/account-sdk/src/interface/public-utilities/prolink/createProlinkUrl.ts and the adjacent createProlinkUrl.test.ts. Run the existing unit test, then cover the case where additionalQueryParams includes p alongside a normal parameter. Done means the prolink argument remains the p query value while other additional parameters still work.

Written by the indexing model from the issue text.