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.

test/pantallas/pantalla_buscar_shimmer_test.dart

@@ -21,6 +21,17 @@ import '../helpers/fakes.dart';
 /// provider from the start, not just `EstadoBusqueda`. A non-empty query is
 /// entered first so the screen leaves the landing state and reaches the
 /// search-results branch this test actually targets.
+///
+/// Bugfix (surfaced while landing audit item 18's "Explorar por" grid):
+/// on the FIRST pump here (landing state, empty `EstadoRadio`, default
+/// narrow test viewport, no `_setLargeSurfaceSize`), `_gridEmisoras`
+/// renders its empty-state branch — `PluriEmptyState`, in a fixed
+/// `SizedBox(height: 260)`. That widget's own Column overflowed by 10px
+/// at this width with real ARB copy (`pluri_premium_widgets.dart`,
+/// fixed by wrapping it in a `SingleChildScrollView` — a no-op whenever
+/// content already fits, which is every real device width). This test IS
+/// the end-to-end regression guard for that fix: it renders the exact
+/// scenario that surfaced it.
 class _BusquedaCargando extends EstadoBusqueda {
   _BusquedaCargando() : super(radio: FakeServicioRadio());