feat: Add textarea form element

CLAUDE.md

@@ -99,7 +99,7 @@ Application Form
             └── Form Elements (FormElement)
                  ├── id (UUID - generated by backend)
                  ├── reference (string - custom key like "art_der_massnahme")
-                 ├── type (SELECT, CHECKBOX, RADIOBUTTON, TEXTFIELD, SWITCH, TITLE_BODY_TEXTFIELDS)
+                 ├── type (SELECT, CHECKBOX, RADIOBUTTON, TEXTFIELD, TEXTAREA, SWITCH, TITLE_BODY_TEXTFIELDS, DATE)
                  ├── title, description
                  ├── options (FormOption[])
                  │    ├── value, label
@@ -227,7 +227,7 @@ Form elements can be conditionally shown or hidden based on the values of other
   "reference": "testphase_zeitraum",
   "title": "Testphase Zeitraum",
   "description": "Zeitraum der Testphase",
-  "type": "TEXTFIELD",
+  "type": "TEXTAREA",
   "options": [
     {
       "value": "",