feat(prime): wire prompt_override into the daily prep prompt

The prime_schedules.prompt_override column existed end-to-end but was
never populated or read. DailyPrepPrompt.BuildPrompt now takes an
optional override and appends it as an extra paragraph after the fixed
prompt (additive, never a replacement, so a user can't disable the
required get_daily_prep_candidates/set_my_day flow). PrimeRunner passes
schedule.PromptOverride through. The Prime tab in Settings now has a
multiline field per schedule wired to a new PromptOverride property on
PrimeScheduleRowViewModel (blank persists as null).

src/ClaudeDo.Localization/locales/de.json

@@ -75,6 +75,8 @@
       "addSchedule": "+ Zeitplan hinzufügen",
       "removeScheduleTip": "Zeitplan entfernen",
       "dailyPrepMaxTasks": "Max. Aufgaben pro Tag",
+      "promptOverrideLabel": "Prompt-Zusatz",
+      "promptOverridePlaceholder": "Zusätzliche Anweisungen, die für diesen Zeitplan an den Prime-Prompt angehängt werden (optional)",
       "dayMo": "Mo",
       "dayTu": "Di",
       "dayWe": "Mi",