fix(favoritos): restyle the dashed custom-station CTA to spec

Tier 4 visual fidelity, audit 4.5 (t4 line 235): the border and the
label/icon now use the prototype's two DIFFERENT opacities
(rgba(255,255,255,.16) stroke vs rgba(242,247,250,.6) text/icon,
previously one shared 50% colour for both), padding is a uniform 14
instead of symmetric(18,16), and the icon is a plain add glyph at 20px
instead of add_circle_outline_rounded.

lib/pantallas/pantalla_favoritos.dart

@@ -474,24 +474,37 @@ class _CtaEmisoraPersonalizada extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final l10n = AppLocalizations.of(context);
-    final color = Theme.of(
+    // Audit 4.5 (t4 line 235): the border and the label/icon are TWO
+    // different opacities in the prototype — `rgba(255,255,255,.16)` for
+    // the dashed stroke, `rgba(242,247,250,.6)` for the text/icon — not one
+    // shared 50% colour for both.
+    final colorBorde = Colors.white.withValues(alpha: 0.16);
+    final colorTexto = Theme.of(
       context,
-    ).colorScheme.onSurface.withValues(alpha: 0.5);
+    ).colorScheme.onSurface.withValues(alpha: 0.6);
     return CustomPaint(
-      painter: _DashedBorderPainter(color: color),
+      painter: _DashedBorderPainter(color: colorBorde),
       child: Material(
         color: Colors.transparent,
         child: InkWell(
           borderRadius: BorderRadius.circular(16),
           onTap: onTap,
           child: Padding(
-            padding: const EdgeInsets.symmetric(vertical: 18, horizontal: 16),
+            key: const Key('custom-station-cta-padding'),
+            padding: const EdgeInsets.all(14),
             child: Row(
               mainAxisAlignment: MainAxisAlignment.center,
               children: [
-                Icon(Icons.add_circle_outline_rounded, color: color),
+                Icon(Icons.add_rounded, size: 20, color: colorTexto),
                 const SizedBox(width: 8),
-                Text(l10n.customStationsAddCta, style: TextStyle(color: color)),
+                Text(
+                  l10n.customStationsAddCta,
+                  style: TextStyle(
+                    fontSize: 13.5,
+                    fontWeight: FontWeight.w800,
+                    color: colorTexto,
+                  ),
+                ),
               ],
             ),
           ),