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.dart

@@ -322,6 +322,30 @@ abstract class AppLocalizations {
   /// **'Alle'**
   String get taskFormFrequencyEvery;
 
+  /// No description provided for @frequencyShortcutDaily.
+  ///
+  /// In de, this message translates to:
+  /// **'Täglich'**
+  String get frequencyShortcutDaily;
+
+  /// No description provided for @frequencyShortcutWeekly.
+  ///
+  /// In de, this message translates to:
+  /// **'Wöchentlich'**
+  String get frequencyShortcutWeekly;
+
+  /// No description provided for @frequencyShortcutBiweekly.
+  ///
+  /// In de, this message translates to:
+  /// **'Alle 2 Wochen'**
+  String get frequencyShortcutBiweekly;
+
+  /// No description provided for @frequencyShortcutMonthly.
+  ///
+  /// In de, this message translates to:
+  /// **'Monatlich'**
+  String get frequencyShortcutMonthly;
+
   /// No description provided for @taskFormFrequencyUnitDays.
   ///
   /// In de, this message translates to: