feat(09-01): rework frequency picker with shortcut chips and freeform picker

- Replace 10-chip grid + hidden Custom mode with 4 shortcut chips (Täglich, Wöchentlich, Alle 2 Wochen, Monatlich)
- Always-visible freeform 'Alle [N] [Tage/Wochen/Monate]' picker row below chips
- Bidirectional sync: tapping chip populates picker; editing picker recalculates chip highlight
- _resolveFrequency() now reads exclusively from picker (single source of truth)
- Edit mode correctly loads all 8 IntervalType values including quarterly and yearly
- Add l10n keys frequencyShortcutDaily/Weekly/Biweekly/Monthly to app_de.arb

lib/l10n/app_localizations_de.dart

@@ -128,6 +128,18 @@ class AppLocalizationsDe extends AppLocalizations {
   @override
   String get taskFormFrequencyEvery => 'Alle';
 
+  @override
+  String get frequencyShortcutDaily => 'Täglich';
+
+  @override
+  String get frequencyShortcutWeekly => 'Wöchentlich';
+
+  @override
+  String get frequencyShortcutBiweekly => 'Alle 2 Wochen';
+
+  @override
+  String get frequencyShortcutMonthly => 'Monatlich';
+
   @override
   String get taskFormFrequencyUnitDays => 'Tage';