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/settings.rs

@@ -1,4 +1,4 @@
-//! TRACES: UR-023, UR-031, UR-032, UR-033 | DR-034, DR-035, DR-036, DR-048
+//! TRACES: UR-023, UR-027, UR-031, UR-032, UR-033 | DR-030, DR-034, DR-035, DR-036, DR-048, IR-020
 
 use serde::{Deserialize, Serialize};
 
@@ -26,6 +26,67 @@ impl VolumeLevel {
     }
 }
 
+/// Centre frequencies (Hz) of the fixed 10-band ISO equalizer. The band count
+/// and layout are a property of the audio engine, not the UI — presets and the
+/// MPV filter are defined against these bands. See docs/specs/audio-equalizer.md.
+///
+/// TRACES: UR-027 | DR-030, IR-020
+pub const EQ_BANDS: [f32; 10] = [
+    31.0, 62.0, 125.0, 250.0, 500.0, 1000.0, 2000.0, 4000.0, 8000.0, 16000.0,
+];
+/// Minimum per-band gain in dB.
+pub const EQ_GAIN_MIN: f32 = -12.0;
+/// Maximum per-band gain in dB.
+pub const EQ_GAIN_MAX: f32 = 12.0;
+
+/// Built-in equalizer presets. A preset *is* a gain curve defined by the band
+/// layout above (a domain concept), not a mere label — the curve numbers live
+/// in Rust so the frontend never encodes the taxonomy.
+///
+/// TRACES: UR-027 | DR-030
+#[derive(specta::Type, Debug, Clone, Copy, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub enum EqPreset {
+    Flat,
+    Rock,
+    Pop,
+    Jazz,
+    Classical,
+    BassBoost,
+    TrebleBoost,
+    Vocal,
+}
+
+impl EqPreset {
+    /// All presets, for enumerating the curve table across the IPC boundary.
+    pub const ALL: [EqPreset; 8] = [
+        EqPreset::Flat,
+        EqPreset::Rock,
+        EqPreset::Pop,
+        EqPreset::Jazz,
+        EqPreset::Classical,
+        EqPreset::BassBoost,
+        EqPreset::TrebleBoost,
+        EqPreset::Vocal,
+    ];
+
+    /// The 10-band gain curve (dB) for this preset, one entry per [`EQ_BANDS`].
+    /// Curves are conservative (within ±8 dB) so presets stack safely with the
+    /// player volume. Bands: 31 62 125 250 500 1k 2k 4k 8k 16k.
+    pub fn gains(&self) -> [f32; 10] {
+        match self {
+            EqPreset::Flat => [0.0; 10],
+            EqPreset::Rock => [5.0, 4.0, 3.0, 1.0, -1.0, -1.0, 1.0, 3.0, 4.0, 5.0],
+            EqPreset::Pop => [-1.0, 0.0, 2.0, 4.0, 5.0, 4.0, 2.0, 0.0, -1.0, -1.0],
+            EqPreset::Jazz => [3.0, 2.0, 1.0, 2.0, -1.0, -1.0, 0.0, 1.0, 2.0, 3.0],
+            EqPreset::Classical => [4.0, 3.0, 2.0, 1.0, -1.0, -1.0, 0.0, 2.0, 3.0, 4.0],
+            EqPreset::BassBoost => [7.0, 6.0, 5.0, 3.0, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0],
+            EqPreset::TrebleBoost => [0.0, 0.0, 0.0, 0.0, 0.0, 1.0, 3.0, 5.0, 6.0, 7.0],
+            EqPreset::Vocal => [-2.0, -1.0, 0.0, 2.0, 4.0, 5.0, 4.0, 2.0, 0.0, -1.0],
+        }
+    }
+}
+
 /// Audio playback settings
 #[derive(specta::Type, Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
@@ -38,6 +99,18 @@ pub struct AudioSettings {
     pub normalize_volume: bool,
     /// Target volume level for normalization
     pub volume_level: VolumeLevel,
+    /// Enable the graphic equalizer. When false, no EQ filter is applied.
+    #[serde(default)]
+    pub equalizer_enabled: bool,
+    /// Per-band gains in dB, one per [`EQ_BANDS`]. Normalised to 10 entries and
+    /// clamped to [`EQ_GAIN_MIN`, `EQ_GAIN_MAX`] via [`Self::with_equalizer_normalised`].
+    #[serde(default = "default_eq_bands")]
+    pub equalizer_bands: Vec<f32>,
+}
+
+/// Flat 10-band curve — the default equalizer state.
+fn default_eq_bands() -> Vec<f32> {
+    vec![0.0; EQ_BANDS.len()]
 }
 
 impl Default for AudioSettings {
@@ -47,6 +120,8 @@ impl Default for AudioSettings {
             gapless_playback: true,
             normalize_volume: false,
             volume_level: VolumeLevel::Normal,
+            equalizer_enabled: false,
+            equalizer_bands: default_eq_bands(),
         }
     }
 }
@@ -57,6 +132,20 @@ impl AudioSettings {
         self.crossfade_duration = self.crossfade_duration.clamp(0.0, 12.0);
         self
     }
+
+    /// Normalise the equalizer band vector to exactly [`EQ_BANDS`]`.len()`
+    /// entries (pad with 0 dB / truncate) and clamp each gain to the valid
+    /// range. Guards against malformed persisted or IPC input.
+    ///
+    /// TRACES: UR-027 | DR-030
+    pub fn with_equalizer_normalised(mut self) -> Self {
+        let n = EQ_BANDS.len();
+        self.equalizer_bands.resize(n, 0.0);
+        for g in &mut self.equalizer_bands {
+            *g = g.clamp(EQ_GAIN_MIN, EQ_GAIN_MAX);
+        }
+        self
+    }
 }
 
 /// Video playback settings
@@ -101,6 +190,95 @@ mod tests {
         assert!(settings.gapless_playback);
         assert!(!settings.normalize_volume);
         assert_eq!(settings.volume_level, VolumeLevel::Normal);
+        // Equalizer defaults: disabled and flat.
+        assert!(!settings.equalizer_enabled);
+        assert_eq!(settings.equalizer_bands, vec![0.0; EQ_BANDS.len()]);
+    }
+
+    /// EQ presets each return one gain per band; Flat is all zeros.
+    ///
+    /// TRACES: UR-027 | DR-030 | UT-079
+    #[test]
+    fn test_eq_preset_curves() {
+        for preset in EqPreset::ALL {
+            assert_eq!(
+                preset.gains().len(),
+                EQ_BANDS.len(),
+                "preset {:?} must have one gain per band",
+                preset
+            );
+            // Every preset stays within the advertised gain range.
+            for g in preset.gains() {
+                assert!(
+                    (EQ_GAIN_MIN..=EQ_GAIN_MAX).contains(&g),
+                    "preset {:?} gain {} out of range",
+                    preset,
+                    g
+                );
+            }
+        }
+        assert_eq!(EqPreset::Flat.gains(), [0.0; 10]);
+        // Bass boost lifts the low bands and leaves the top flat.
+        let bass = EqPreset::BassBoost.gains();
+        assert!(bass[0] > 0.0 && bass[9] == 0.0);
+    }
+
+    /// `with_equalizer_normalised` clamps out-of-range gains and forces the
+    /// band vector to exactly EQ_BANDS.len() (pad short, truncate long).
+    ///
+    /// TRACES: UR-027 | DR-030 | UT-080
+    #[test]
+    fn test_eq_normalisation() {
+        // Out-of-range gains are clamped.
+        let s = AudioSettings {
+            equalizer_bands: vec![100.0, -100.0, 3.0],
+            ..Default::default()
+        }
+        .with_equalizer_normalised();
+        assert_eq!(s.equalizer_bands.len(), EQ_BANDS.len());
+        assert_eq!(s.equalizer_bands[0], EQ_GAIN_MAX);
+        assert_eq!(s.equalizer_bands[1], EQ_GAIN_MIN);
+        assert_eq!(s.equalizer_bands[2], 3.0);
+        // Short vector padded with 0 dB.
+        assert_eq!(s.equalizer_bands[9], 0.0);
+
+        // Over-long vector truncated.
+        let long = AudioSettings {
+            equalizer_bands: vec![1.0; 20],
+            ..Default::default()
+        }
+        .with_equalizer_normalised();
+        assert_eq!(long.equalizer_bands.len(), EQ_BANDS.len());
+    }
+
+    /// Old persisted JSON without the EQ fields loads as disabled + flat.
+    ///
+    /// TRACES: UR-027 | DR-030 | UT-081
+    #[test]
+    fn test_audio_settings_eq_backward_compat() {
+        let json = r#"{"crossfadeDuration":0.0,"gaplessPlayback":true,"normalizeVolume":false,"volumeLevel":"normal"}"#;
+        let parsed: AudioSettings = serde_json::from_str(json).unwrap();
+        assert!(!parsed.equalizer_enabled);
+        assert_eq!(parsed.equalizer_bands, vec![0.0; EQ_BANDS.len()]);
+    }
+
+    /// EQ fields serialize as camelCase and round-trip.
+    ///
+    /// TRACES: UR-027 | DR-030 | UT-082
+    #[test]
+    fn test_audio_settings_eq_serialization() {
+        let settings = AudioSettings {
+            equalizer_enabled: true,
+            equalizer_bands: EqPreset::Rock.gains().to_vec(),
+            ..Default::default()
+        };
+        let json = serde_json::to_string(&settings).unwrap();
+        assert!(json.contains("\"equalizerEnabled\":true"));
+        assert!(json.contains("\"equalizerBands\":"));
+
+        let parsed: AudioSettings = serde_json::from_str(&json).unwrap();
+        assert!(parsed.equalizer_enabled);
+        assert_eq!(parsed.equalizer_bands, EqPreset::Rock.gains().to_vec());
     }
 
     #[test]
@@ -134,6 +312,7 @@ mod tests {
             gapless_playback: true,
             normalize_volume: true,
             volume_level: VolumeLevel::Loud,
+            ..Default::default()
         };
 
         let json = serde_json::to_string(&settings).unwrap();