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/l10n/gen/app_localizations.dart

@@ -1438,6 +1438,36 @@ abstract class AppLocalizations {
   /// **'Géneros'**
   String get genresTitle;
 
+  /// No description provided for @exploreByTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Explorar por'**
+  String get exploreByTitle;
+
+  /// No description provided for @exploreTrendingTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Tendencias'**
+  String get exploreTrendingTitle;
+
+  /// No description provided for @exploreTrendingSubtitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Hoy'**
+  String get exploreTrendingSubtitle;
+
+  /// No description provided for @exploreNewTitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Novedades'**
+  String get exploreNewTitle;
+
+  /// No description provided for @exploreNewSubtitle.
+  ///
+  /// In es, this message translates to:
+  /// **'Esta semana'**
+  String get exploreNewSubtitle;
+
   /// No description provided for @offlineBannerTitle.
   ///
   /// In es, this message translates to: