URLSearchParams.toString() encodes spaces as +, but Bilibili's WBI
signature verification expects %20. This mismatch causes search
queries with spaces (e.g. "亚马逊 滞销产品") to fail with
TypeError: Failed to fetch due to CORS-blocked error responses.
Fixes#125
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>