fix(visualizador): add a discrete-bar rendering mode

Audit 1.7/2.5 (t4 lines 66-68, 120-122): the prototype draws 30
discrete bottom-anchored bars (radius 2, vertical gradient); the
build's VisualizadorAudio painted a single continuous oscilloscope
stroke, so its `barras: 30` parameter never produced bars.

Adds an opt-in `barrasDiscretas` mode (default false, byte-identical
continuous rendering preserved for any other caller) plus a
`gradienteFinAlpha` knob for the two screens' differing gradient
end-alpha (.3 vs .45). Wires it into the Escuchar hero and the full
player, correcting the player's bar count/height to match the
prototype (26->30 bars, 46->40px) at the same time.

lib/pantallas/pantalla_inicio.dart

@@ -291,6 +291,9 @@ class _EscucharHero extends StatelessWidget {
                         barras: 30,
                         altura: 26,
                         color: context.pluriTokens.liveGreen,
+                        // Audit 1.7 (t4 lines 66-68): 30 discrete
+                        // bottom-anchored bars, not a continuous stroke.
+                        barrasDiscretas: true,
                       ),
                     ],
                   ),