fix(onboarding): surface errors and supply default schedule (#87)

Issue #87 (kevinpapst): medications added during onboarding were
never visible afterwards. Root cause: the createMedication schema
requires schedules.min(1) ("Mindestens ein Zeitfenster"), but the
onboarding wizard sent schedules:[]. The 422 was silently swallowed
and the user was redirected to the dashboard as if everything had
worked.

This patch is the minimal-impact fix. The proper fix lives in Phase
3.5 of the audit plan, which replaces the wizard entirely with a
dashboard-first empty-state flow. Until then:

- finishOnboarding() wraps each call in a try/catch, checks
  response.ok explicitly, and surfaces a sonner toast on failure.
  setSaving(false) on error so the user can retry instead of being
  stuck on a spinner.
- Each medication is sent with a default schedule (08:00-09:00) so
  it is actually persisted. A new medScheduleHint i18n key is shown
  under the meds list while the user is in the wizard, so the
  default schedule is not surprising.
- Errors are translated per stage (errorProfile, errorMedication
  with {name}, errorTargets) plus an errorGeneric fallback. en.json
  and de.json kept symmetric.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

messages/de.json

@@ -819,7 +819,12 @@
     "targetsDescription": "Setze optionale Zielwerte für Blutdruck und Gewicht.",
     "targetSys": "Systolisch (mmHg)",
     "targetDia": "Diastolisch (mmHg)",
-    "targetWeight": "Zielgewicht (kg)"
+    "targetWeight": "Zielgewicht (kg)",
+    "errorProfile": "Dein Profil konnte nicht gespeichert werden. Bitte erneut versuchen.",
+    "errorMedication": "Das Medikament „{name}\" konnte nicht angelegt werden. Bitte erneut versuchen oder später unter „Medikamente\" hinzufügen.",
+    "errorTargets": "Deine Zielwerte konnten nicht gespeichert werden. Bitte erneut versuchen.",
+    "errorGeneric": "Beim Abschluss des Setups ist etwas schiefgelaufen. Bitte erneut versuchen.",
+    "medScheduleHint": "Wir legen ein Standard-Erinnerungsfenster an (08:00–09:00 täglich). Du kannst es jederzeit unter „Medikamente\" anpassen."
   },
   "settings": {
     "title": "Einstellungen",