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/app_ru.arb

@@ -306,6 +306,11 @@
   "detectAction": "Определить",
   "liveRadar": "Радар в эфире",
   "genresTitle": "Жанры",
+  "exploreByTitle": "Обзор по",
+  "exploreTrendingTitle": "В тренде",
+  "exploreTrendingSubtitle": "Сегодня",
+  "exploreNewTitle": "Новинки",
+  "exploreNewSubtitle": "На этой неделе",
   "offlineBannerTitle": "Нет подключения",
   "retryAction": "Повторить",
   "noStationsAvailable": "Нет доступных станций",