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>

BUILD.md

@@ -0,0 +1,83 @@
+# Building qobuz-qt
+
+## Dependencies
+
+### Debian / Ubuntu
+```bash
+sudo apt install -y \
+    cmake \
+    ninja-build \
+    qt6-base-dev \
+    qt6-svg-dev \
+    libssl-dev \
+    libasound2-dev \
+    libdbus-1-dev \
+    pkg-config \
+    curl
+
+# Rust toolchain
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source "$HOME/.cargo/env"
+```
+
+### Arch Linux
+```bash
+sudo pacman -S cmake ninja qt6-base qt6-svg openssl alsa-lib rust
+```
+
+### Fedora
+```bash
+sudo dnf install cmake ninja-build qt6-qtbase-devel qt6-qtsvg-devel \
+    openssl-devel alsa-lib-devel rust cargo
+```
+
+## Build
+
+```bash
+cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
+cmake --build build -j$(nproc)
+```
+
+The first build will take longer as it compiles the Rust backend via
+[Corrosion](https://github.com/corrosion-rs/corrosion).
+
+## Run
+
+```bash
+./build/qobuz-qt
+```
+
+On first launch a login dialog appears.  Enter your Qobuz email and password.
+Credentials are stored via `QSettings` in the standard user config directory.
+
+## Audio Quality
+
+Open **File → Settings** to choose the preferred streaming quality:
+- Hi-Res 24-bit/192 kHz (format 27)
+- Hi-Res 24-bit/96 kHz  (format 7)
+- CD 16-bit              (format 6)  ← default
+- MP3 320 kbps           (format 5)
+
+## Architecture
+
+```
+qobuz-qt/
+├── CMakeLists.txt          Root build file (uses Corrosion for Rust)
+├── Cargo.toml              Cargo workspace
+├── rust/                   Rust backend (static library)
+│   ├── Cargo.toml
+│   ├── include/
+│   │   └── qobuz_backend.h C header for FFI
+│   └── src/
+│       ├── lib.rs          extern "C" API + Tokio runtime
+│       ├── api/            Qobuz HTTP API client (reqwest + tokio)
+│       └── player/         Audio playback (Symphonia decoder + CPAL output)
+└── src/                    C++ / Qt 6 frontend
+    ├── backend/            Qt QObject wrapper around the Rust library
+    ├── view/               Toolbar, main content, side panel
+    ├── list/               Track list, library sidebar
+    ├── model/              TrackListModel (QAbstractTableModel)
+    ├── dialog/             Login, Settings dialogs
+    ├── widget/             VolumeButton, ClickableSlider
+    └── util/               AppSettings (QSettings), Icon helpers
+```