fix: 音源导入去重 (#2681)

* fix: 音源导入添加去重逻辑
修复音源导入时未对重复数据进行去重的问题。现在会根据url去重，避免导入重复的音源。
Fixes lyswhut/lx-music-desktop#2649

* fix: check script content instead of name for dedup

Allow duplicate source names (intentional behavior).
Only reject import when script content is identical to an existing
source, and report which source name conflicts.