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/commands/player/settings.rs

@@ -1,12 +1,12 @@
 //! Audio and video playback settings commands.
 //!
-//! TRACES: UR-022, UR-031, UR-032, UR-033 | DR-025, DR-034, DR-035, DR-036
+//! TRACES: UR-022, UR-027, UR-031, UR-032, UR-033 | DR-025, DR-030, DR-034, DR-035, DR-036, IR-020
 
 use tauri::State;
 
 use super::{PlayerStateWrapper, VideoSettingsWrapper};
 use crate::player::AutoplaySettings;
-use crate::settings::{AudioSettings, VideoSettings};
+use crate::settings::{AudioSettings, EqPreset, VideoSettings};
 
 #[tauri::command]
 #[specta::specta]
@@ -14,13 +14,32 @@ pub async fn player_set_audio_settings(
     player: State<'_, PlayerStateWrapper>,
     settings: AudioSettings,
 ) -> Result<AudioSettings, String> {
+    // Validate/normalise domain values before applying: clamp crossfade to its
+    // range and normalise the equalizer band vector (length + gain clamps).
+    let validated = settings
+        .with_crossfade_clamped()
+        .with_equalizer_normalised();
     let mut controller = player.0.lock().await;
     controller
-        .set_audio_settings(&settings)
+        .set_audio_settings(&validated)
         .map_err(|e| e.to_string())?;
     Ok(controller.audio_settings())
 }
 
+/// The built-in equalizer presets and their per-band gain curves (dB), for the
+/// settings UI. The curve numbers are domain data defined by the band layout,
+/// so the frontend reads them here rather than encoding them.
+///
+/// TRACES: UR-027 | DR-030
+#[tauri::command]
+#[specta::specta]
+pub async fn player_get_eq_presets() -> Result<Vec<(EqPreset, Vec<f32>)>, String> {
+    Ok(EqPreset::ALL
+        .iter()
+        .map(|p| (*p, p.gains().to_vec()))
+        .collect())
+}
+
 #[tauri::command]
 #[specta::specta]
 pub async fn player_get_audio_settings(