fix: handle empty HRV field in vitals form

- Only include fields in payload if they have values
- Prevents sending empty strings to backend (Pydantic validation error)
- Applies to both create and update operations

Error was: 'Input should be a valid integer, unable to parse string as an integer'

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/VitalsPage.jsx

@@ -122,9 +122,12 @@ export default function VitalsPage() {
     setError(null)
 
     try {
-      const payload = { ...form }
-      if (payload.resting_hr) payload.resting_hr = parseInt(payload.resting_hr)
-      if (payload.hrv) payload.hrv = parseInt(payload.hrv)
+      const payload = { date: form.date }
+
+      // Only include fields if they have values
+      if (form.resting_hr) payload.resting_hr = parseInt(form.resting_hr)
+      if (form.hrv) payload.hrv = parseInt(form.hrv)
+      if (form.note) payload.note = form.note
 
       if (!payload.resting_hr && !payload.hrv) {
         setError('Mindestens Ruhepuls oder HRV muss angegeben werden')
@@ -150,9 +153,13 @@ export default function VitalsPage() {
 
   const handleUpdate = async () => {
     try {
-      const payload = { ...editing }
-      if (payload.resting_hr) payload.resting_hr = parseInt(payload.resting_hr)
-      if (payload.hrv) payload.hrv = parseInt(payload.hrv)
+      const payload = {}
+
+      // Only include fields if they have values
+      if (editing.date) payload.date = editing.date
+      if (editing.resting_hr) payload.resting_hr = parseInt(editing.resting_hr)
+      if (editing.hrv) payload.hrv = parseInt(editing.hrv)
+      if (editing.note) payload.note = editing.note
 
       await api.updateVitals(editing.id, payload)
       setEditing(null)