fix(vacaciones): header Add action, info banner, dashed CTA, collapse

Audit 9b.1 (t4:446): a solid brand-teal "Add" header action -- the
prototype's own mid-page CTA (audit 9b.6, still present, now dashed)
is a SECOND, additional entry point, not a replacement.

Audit 9b.2 (t4:448): the teal explanatory banner ("alarms marked
pause-during-vacations won't ring...") is now ALWAYS visible -- never
rendered before. New ARB key vacationExplainerBanner, all 13 locales.

Audit 9b.6 (t4:487): the bottom CTA is now dashed-border with a
date_range glyph -- was a solid OutlinedButton with an add glyph.
Reuses the dashed-painter shape already established in
pantalla_favoritos.dart's custom-station CTA (audit 4.5), duplicated
rather than shared.

Audit 9b.7 (t4:489): "Rangos pasados" is now a collapsible row --
icon, title, count, chevron -- COLLAPSED by default, expanding on tap.
Was always fully expanded inline. Updated the pre-existing widget test
to tap-then-assert instead of asserting immediate visibility.

Item 9b.3 (the eyebrow literally reading "EN CURSO") is NOT
implemented: the eyebrow STYLING is already correct (audit 9b.4), and
the residual copy gap is a shared ARB string
(vacationSummaryActiveCountdown) also used compactly in
pantalla_alarmas.dart's vacation summary row -- diverging its wording
just for this screen's eyebrow, or forcing a shoutier tone into that
other compact usage, is not worth it for a trivial-rated copy nuance.

lib/l10n/app_ru.arb

@@ -788,6 +788,7 @@
       "times": {}
     }
   },
+  "vacationExplainerBanner": "В эти периоды будильники с пометкой «пауза на время отпуска» не звонят. Будильники с пометкой «звонить всегда» это не затрагивает.",
   "vacationNoActiveRangeHint": "Сейчас нет активного периода отпуска.",
   "vacationPastSectionTitle": "Прошедшие периоды",
   "vacationRangesCount": "{count} периодов",