Add patient dialog: use a date picker for date fields

Install COSS Calendar (@coss/calendar, react-day-picker) and add a co-located
DatePicker (Popover + Button trigger + Calendar) in patient-form-dialog. It
reads/writes the same formatted date string the form already uses (formatDate/
parseDate), so the Patient model is unchanged. Replace the free-text Visit 'Date'
and Problem 'Since' inputs with the picker.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -34,6 +34,7 @@
     "next": "16.2.6",
     "next-themes": "^0.4.6",
     "react": "19.2.4",
+    "react-day-picker": "^10.0.1",
     "react-dom": "19.2.4",
     "react-i18next": "^17.0.8",
     "react-jsx-parser": "^2.4.1",