Feat: Adding real-time audio visualisation

[Narqulie]

No description provided.

[Narqulie]

Implements the frequency visualization requested in #689.

What this does

Adds an opt-in bar chart to the playback window showing real-time frequency content of the music being played — 64 log-scale bands from bass (left) to treble (right), with a smooth decay so bars fade out gradually rather
than snapping to silence.

How it works

Rather than using the deprecated Spotify Audio Analysis API, this taps directly into the librespot audio stream. A VisualizationSink wraps the real audio backend, intercepts decoded PCM samples, and runs an FFT using rustfft
(https://crates.io/crates/rustfft). Results are written to a shared buffer that the UI reads each render frame.

Key details:

• Sliding window FFT (FFT_SIZE=1024, HOP_SIZE=256) gives ~5.8ms update rate — well ahead of the ~32ms UI refresh
• Hann windowing to reduce spectral leakage
• Bars rise instantly on loud transients, decay slowly with a configurable factor
• Only active with the streaming feature; no-op and zero overhead otherwise
• Disabled by default — opt in via enable_audio_visualization = true in app.toml

To try it

Add to your ~/.config/spotify-player/app.toml:

enable_audio_visualization = true

Then build with the streaming feature enabled (the default).