Show existing album count in sort preview

Summary

• Queries sorted_albums table to show how many albums already exist for the destination artist+category in the sort preview
• Helps catch artist name misspellings (e.g. 藤井風 vs 藤井 風) — seeing "0 existing albums" signals you may be creating a duplicate artist directory
• Count updates in real-time as you edit the artist name (via existing 300ms debounced auto-preview)
• Skipped for Soundtrack category (no artist directory)

Test plan

• Sort preview shows existing album count below target path
• Changing artist name updates the count
• Soundtrack category hides the count
• Build passes

🤖 Generated with Claude Code