feat(audio): graphic equalizer with presets and custom bands

Adds a 10-band graphic equalizer to AudioSettings (enabled flag +
per-band dB gains, normalised to 10 entries and clamped to range).
Presets return gain curves; the settings page gains EQ UI. libmpv
applies the filter on Linux (Android parity pending). Old persisted
settings without EQ fields load as disabled + flat.

Also includes the requirements/traceability/ux-flows doc updates for
this feature and the home long-press routing (UR-058/DR-087).

TRACES: UR-027 | IR-020, DR-030 | UT-079, UT-080, UT-081, UT-082

src-tauri/src/player/backend.rs

@@ -332,6 +332,7 @@ mod tests {
             gapless_playback: false,
             normalize_volume: true,
             volume_level: VolumeLevel::Loud,
+            ..Default::default()
         };
 
         backend.set_audio_settings(&settings).unwrap();