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_de.arb

@@ -48,6 +48,10 @@
   "taskFormFrequencyLabel": "Wiederholung",
   "taskFormFrequencyCustom": "Benutzerdefiniert",
   "taskFormFrequencyEvery": "Alle",
+  "frequencyShortcutDaily": "Täglich",
+  "frequencyShortcutWeekly": "Wöchentlich",
+  "frequencyShortcutBiweekly": "Alle 2 Wochen",
+  "frequencyShortcutMonthly": "Monatlich",
   "taskFormFrequencyUnitDays": "Tage",
   "taskFormFrequencyUnitWeeks": "Wochen",
   "taskFormFrequencyUnitMonths": "Monate",