fix(buscar): build the 2x2 "Explorar por" grid, add Novedades

Audit 3.2 (t4 lines 162-171): the prototype groups Países, Géneros,
Tendencias and the entirely-missing Novedades into one 2x2 entry-point
grid. The build had them as three unrelated always-visible widgets
(a Países ListTile, a Géneros FilterChip Wrap, a Tendencias ActionChip
strip) and no Novedades entry at all.

Replaces all three with a single grid section, capability-preserving:
Países still pushes PantallaPaises; Géneros and Tendencias now open
their exact existing content in a picker sheet instead of always-on-
screen (Géneros auto-closes on selection, matching this screen's other
single-choice filter sheets; Tendencias stays open, a browse list, not
a filter). Novedades re-triggers the existing discovery refresh, since
no distinct "new stations" feed exists anywhere in the domain.

New ARB key set (exploreByTitle/exploreTrendingTitle/
exploreTrendingSubtitle/exploreNewTitle/exploreNewSubtitle) translated
across all 13 locales with zero anti-copy allowlist entries needed.

Also fixes a pre-existing PluriEmptyState overflow this restructure
exposed (unrelated to the grid itself, confirmed via isolated repro):
wraps its Column in a SingleChildScrollView so a too-tall title/
subtitle scrolls instead of throwing a hard RenderFlex overflow.

lib/widgets/pluri_premium_widgets.dart

@@ -74,31 +74,42 @@ class PluriEmptyState extends StatelessWidget {
         child: PluriGlassSurface(
           borderRadius: BorderRadius.circular(t.radiusLg + 10),
           padding: const EdgeInsets.all(28),
-          child: Column(
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              PluriIcon(
-                glyph: glyph,
-                variant: PluriIconVariant.activeGlow,
-                size: 58,
-              ),
-              const SizedBox(height: 18),
-              Text(
-                title,
-                textAlign: TextAlign.center,
-                style: theme.textTheme.titleLarge?.copyWith(
-                  fontWeight: FontWeight.w900,
+          // Bugfix: every call site constrains this widget to a fixed
+          // height (`SizedBox(height: 260)`). At a narrow width with
+          // realistic two-line title/subtitle copy, the icon + gaps +
+          // wrapped text can exceed what's actually left after this
+          // surface's own padding — a hard RenderFlex overflow rather
+          // than a graceful clip. `SingleChildScrollView` is a no-op
+          // visually whenever the content already fits (the normal
+          // case, on any real device width this was designed for) and
+          // only engages scrolling in the rare case it does not.
+          child: SingleChildScrollView(
+            child: Column(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                PluriIcon(
+                  glyph: glyph,
+                  variant: PluriIconVariant.activeGlow,
+                  size: 58,
                 ),
-              ),
-              const SizedBox(height: 8),
-              Text(
-                subtitle,
-                textAlign: TextAlign.center,
-                style: theme.textTheme.bodyMedium?.copyWith(
-                  color: theme.colorScheme.onSurface.withValues(alpha: 0.72),
+                const SizedBox(height: 18),
+                Text(
+                  title,
+                  textAlign: TextAlign.center,
+                  style: theme.textTheme.titleLarge?.copyWith(
+                    fontWeight: FontWeight.w900,
+                  ),
                 ),
-              ),
-            ],
+                const SizedBox(height: 8),
+                Text(
+                  subtitle,
+                  textAlign: TextAlign.center,
+                  style: theme.textTheme.bodyMedium?.copyWith(
+                    color: theme.colorScheme.onSurface.withValues(alpha: 0.72),
+                  ),
+                ),
+              ],
+            ),
           ),
         ),
       ),