Move track library off monolithic JSON state

[krwg]

Summary

Full tracks[] in senza-state.json does not scale.

Target milestone: 1.2.0 Dolce
Work branch: 1.2.0-dolce/issue-9-library-db

---

Problem

10k+ tracks cause parse/stringify latency and RAM pressure.

Expected behavior

SQLite/IDB for tracks; state.json for UI prefs only.

Suggested approach

library-db.cjs + migration from existing state.

Files / areas

• new electron/library-db.cjs
• docs/ARCHITECTURE.md

---

Acceptance criteria

• Migration preserves library
• Save mutates index not full JSON

---

Metadata

Field	Value
Priority	P1
Milestone	1.2.0 Dolce
Branch	1.2.0-dolce/issue-9-library-db