feat(ui): a question mark that keeps a section's explanation one tap away

HelpButton, beside a section's or a setting's title: the explanation opens
in a small dialog, under its title where given, rather than standing in the
everyday view (the developer's review of 3 Nov 2026). FieldInfoButton's
tooltip stays for a field's short hint.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_ui/lib/l10n/halkyon_ui_localizations_de.dart

@@ -90,4 +90,7 @@ class HalkyonUiLocalizationsDe extends HalkyonUiLocalizations {
   String colorPickerLabel(String color) {
     return 'Farbe: $color';
   }
+
+  @override
+  String get explain => 'Was ist das?';
 }