Initial implementation of qobuz-qt

- Rust backend (qobuz-backend static lib): Qobuz API client (reqwest/tokio),
  Symphonia audio decoder, CPAL audio output, extern "C" FFI bridge
- Qt 6 frontend mirroring spotify-qt layout: toolbar with playback controls,
  left library dock, central track list, right search panel
- Auth: email/password login with MD5-signed requests; session token persisted
  via QSettings
- Playback: double-click a track → Rust fetches stream URL → Symphonia decodes
  → CPAL outputs to default audio device
- Dark Fusion palette matching spotify-qt feel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/Cargo.toml

@@ -0,0 +1,33 @@
+[package]
+name = "qobuz-backend"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+name = "qobuz_backend"
+crate-type = ["staticlib"]
+
+[dependencies]
+# HTTP client – blocking feature is needed by the audio decoder
+reqwest = { version = "0.12", features = ["json", "rustls-tls", "stream", "blocking"], default-features = false }
+tokio = { version = "1", features = ["full"] }
+
+# Serialization
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+# Audio decoding + output
+symphonia = { version = "0.5", features = ["all"] }
+cpal = { version = "0.15", features = ["jack"] }
+rb = "0.3"
+
+# Misc
+anyhow = "1"
+thiserror = "2"
+md5 = "0.7"
+dirs = "5"
+toml = "0.8"
+
+[profile.release]
+lto = "thin"
+opt-level = 3