feat(eq): add per-device equalizer with 4-level preset resolution

Introduce multi-device EQ support allowing each audio output device
(built-in speaker, wired, USB, individual Bluetooth by MAC) to have
its own equalizer preset, combined with existing per-station presets
for a full station×device matrix.

- Add DispositivoAudio model and ServicioDispositivoAudio interface
- Add Android platform channel (AudioDeviceCallback) for device detection
- Add iOS AudioDevicesPlugin (AVAudioSession route tracking)
- Extend ServicioEcualizador with device and matrix persistence keys
- Implement 4-level resolution: matrix > station > device > global
- Add advanced EQ settings section with feature toggle (off by default)
- Extend export/import to v3 with backward compatibility
- 184 tests passing, zero analyzer issues

lib/pantallas/pantalla_ajustes.dart

@@ -60,6 +60,8 @@ class _AjustesContent extends StatelessWidget {
       children: const [
         _SeccionEcualizador(),
         SizedBox(height: 12),
+        _SeccionEcualizadorAvanzado(),
+        SizedBox(height: 12),
         _SeccionGrabaciones(),
         SizedBox(height: 12),
         _SeccionTimerSueno(),
@@ -652,6 +654,126 @@ class _SeccionEcualizador extends StatelessWidget {
   }
 }
 
+/// "Advanced Equalization Options" settings section (Phase 7, multi-device EQ).
+///
+/// Always shows the feature toggle so the user can discover it. When the
+/// toggle is OFF, the device list is completely absent (not just invisible),
+/// matching the spec scenario "Settings section is absent when toggle is off".
+class _SeccionEcualizadorAvanzado extends StatelessWidget {
+  const _SeccionEcualizadorAvanzado();
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+    final eq = context.watch<EstadoEcualizador>();
+    final multiDeviceEnabled = eq.eqMultiDeviceEnabled;
+    final presetsDispositivo = eq.presetsDispositivo;
+
+    return PluriGlassSurface(
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Row(
+            children: [
+              const Icon(Icons.devices_rounded),
+              const SizedBox(width: 12),
+              Expanded(
+                child: Text(
+                  l10n.advancedEqSectionTitle,
+                  style: Theme.of(context).textTheme.titleMedium,
+                ),
+              ),
+            ],
+          ),
+          // The toggle uses GestureDetector + custom row instead of
+          // SwitchListTile to avoid Material ink assertion inside
+          // PluriGlassSurface's DecoratedBox. The visual result is identical
+          // to SwitchListTile.
+          Padding(
+            padding: const EdgeInsets.symmetric(vertical: 8),
+            child: Row(
+              children: [
+                Expanded(
+                  child: GestureDetector(
+                    behavior: HitTestBehavior.translucent,
+                    onTap:
+                        () => eq.cambiarMultiDeviceEnabled(!multiDeviceEnabled),
+                    child: Column(
+                      crossAxisAlignment: CrossAxisAlignment.start,
+                      children: [
+                        Text(
+                          l10n.advancedEqEnableToggle,
+                          style: Theme.of(context).textTheme.bodyLarge,
+                        ),
+                        const SizedBox(height: 2),
+                        Text(
+                          l10n.advancedEqEnableToggleSubtitle,
+                          style: Theme.of(context).textTheme.bodySmall,
+                        ),
+                      ],
+                    ),
+                  ),
+                ),
+                Switch.adaptive(
+                  value: multiDeviceEnabled,
+                  onChanged:
+                      (habilitado) => eq.cambiarMultiDeviceEnabled(habilitado),
+                ),
+              ],
+            ),
+          ),
+          if (multiDeviceEnabled) ...[
+            const SizedBox(height: 8),
+            Text(
+              l10n.advancedEqKnownDevicesTitle,
+              style: Theme.of(context).textTheme.labelLarge,
+            ),
+            const SizedBox(height: 4),
+            if (presetsDispositivo.isEmpty)
+              Padding(
+                padding: const EdgeInsets.symmetric(vertical: 8),
+                child: Text(
+                  l10n.advancedEqKnownDevicesEmpty,
+                  style: Theme.of(context).textTheme.bodySmall,
+                ),
+              )
+            else
+              for (final entry in presetsDispositivo.entries)
+                Padding(
+                  padding: const EdgeInsets.symmetric(vertical: 6),
+                  child: Row(
+                    children: [
+                      const Icon(Icons.headphones_rounded, size: 20),
+                      const SizedBox(width: 12),
+                      Expanded(
+                        child: Column(
+                          crossAxisAlignment: CrossAxisAlignment.start,
+                          children: [
+                            Text(
+                              entry.key,
+                              maxLines: 1,
+                              overflow: TextOverflow.ellipsis,
+                              style: Theme.of(context).textTheme.bodyMedium,
+                            ),
+                            Text(
+                              l10n.advancedEqDevicePresetLabel(
+                                entry.value.nombre,
+                              ),
+                              style: Theme.of(context).textTheme.bodySmall,
+                            ),
+                          ],
+                        ),
+                      ),
+                    ],
+                  ),
+                ),
+          ],
+        ],
+      ),
+    );
+  }
+}
+
 class _SeccionOrdenListas extends StatelessWidget {
   const _SeccionOrdenListas();