feat(qbz-library): extract local library management to standalone crate

Create qbz-library crate with:
- LibraryScanner: directory scanning for audio files
- MetadataExtractor: audio metadata extraction via lofty
- LibraryDatabase: SQLite persistence with WAL mode
- CueParser: CUE sheet parsing for single-file albums
- Thumbnails: artwork extraction and thumbnail generation
- Tag sidecar: album metadata override files

src-tauri re-exports from qbz-library for backwards compatibility.
Enables TUI/headless clients to manage local libraries without Tauri.

crates/qbz-library/Cargo.toml

@@ -0,0 +1,36 @@
+[package]
+name = "qbz-library"
+version = "0.1.0"
+edition = "2021"
+description = "Local music library management for QBZ - scanning, indexing, metadata extraction"
+authors = ["blitzkriegfc"]
+
+[dependencies]
+# Logging
+log = "0.4"
+
+# Serialization
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+# Error handling
+thiserror = "2"
+
+# Database
+rusqlite = { version = "0.31", features = ["bundled"] }
+
+# Filesystem
+walkdir = "2"
+dirs = "5"
+
+# Audio metadata
+lofty = "0.18"
+
+# Image processing (thumbnails)
+image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }
+
+# File system utilities
+glob = "0.3"
+
+# UUID generation
+uuid = { version = "1", features = ["v4"] }