refactor(favsort): send bilibili requests through httpx

Reuse a pooled httpx client for favsort GET and POST requests, share the raw-request-derived browser headers, and add folder-specific referers for ids, infos and sort. Add transport tests for header parity and anonymous-to-authenticated fallback.

tests/test_favsort_run.py

@@ -74,7 +74,9 @@ def test_sort_preserves_resource_types():
             return [{"id": aid} for aid in order]
 
         favsort.fetch_ids = fetch_ids
-        favsort.post_form = lambda cfg, url, body: bodies.append(body) or {"code": 0}
+        favsort.post_form = (
+            lambda cfg, url, body, referer=None: bodies.append(body) or {"code": 0}
+        )
         args = Args()
         try:
             code = favsort.cmd_run(config(), args)