search: --categories rejects "developer" client-side although POST /v2/search accepts it

### Summary

`firecrawl search --categories developer` exits 1 with `Error: Invalid category "developer". Valid categories: github, research, pdf` before any request is made, but the API accepts the category and returns results tagged `developer`.

### Version

`firecrawl-cli` 1.19.26 (npm), Node 24.12.0, Linux.

### Where

`dist/index.js` line 701 in the published package:

```js
const validCategories = ['github', 'research', 'pdf'];
```

### Reproduction

```
$ firecrawl search "foundationdb queue worker shutdown" --categories developer --limit 2 --json
Error: Invalid category "developer". Valid categories: github, research, pdf
```

The same request against the API succeeds (2026-09-13):

```
POST https://api.firecrawl.dev/v2/search
{"query": "foundationdb queue worker shutdown", "categories": ["developer"], "limit": 2}
-> HTTP 200, success: true, creditsUsed present, results with "category": "developer"
(forums.foundationdb.org pages, which the github category does not return)
```

The hosted MCP server's `firecrawl_search` tool already lists `developer` in its `categories` enum alongside `github`, `research`, and `pdf`.

### Suggested fix

Add `'developer'` to `validCategories`, or drop the client-side list and let the API validate so new categories work without a CLI release.

Start at dist/index.js line 701 and compare the CLI category validation with the hosted MCP categories enum and the API behavior described in the issue. Run the provided firecrawl search command with --categories developer; done means the CLI accepts developer and sends the request successfully instead of rejecting it locally.

Written by the indexing model from the issue text.