fix(alarmas-editor): REPETIR eyebrow, grouped card, volume row

Audit 8.5 (t4:381): the "REPETIR" eyebrow above the weekday circles --
never rendered before. New ARB key alarmRepeatSectionLabel.

Audit 8.6 (t4:392-401): the station picker, volume, fade-in and
vacation toggle now share ONE bordered card with sangred divider lines
-- were four separately-spaced widgets. Restyled _CampoSelectorEmisora
as a flat nav row (icon/bold label/muted value/chevron, matching
FilaAjuste's own convention) so it fits inside the card instead of
drawing its own outlined InputDecoration chrome -- used by BOTH the
primary and fallback (Advanced) station fields. Snooze duration, "use
current station", and the Advanced section stay OUTSIDE the card,
exactly where they were (audit 8.8's own documented deliberate
extras). A transparent Material sits inside the card's coloured box so
the fade-in ListTile and the vacation SwitchListTile still paint their
ink/background correctly (the same class of constraint already
documented for PluriGlassSurface elsewhere in this codebase).

Audit 8.7 (t4:396): the volume row is now a compact 112px track with a
live percentage label -- was a bare full-width Slider with no visible
value. New ArtB key alarmVolumeLabel.

Item 8.8 (extra name/type-selector/snooze-selector/"use current
station"/Advanced fields) stays exactly as documented: deliberate,
required by the hard constraint that the editor keep its date field,
fallback-station picker and sound dropdown.

lib/l10n/gen/app_localizations.dart

@@ -2498,6 +2498,12 @@ abstract class AppLocalizations {
   /// **'REPETIR'**
   String get alarmRepeatSectionLabel;
 
+  /// No description provided for @alarmVolumeLabel.
+  ///
+  /// In es, this message translates to:
+  /// **'Volumen'**
+  String get alarmVolumeLabel;
+
   /// No description provided for @androidReliabilityTitle.
   ///
   /// In es, this message translates to: