fix: relative bold detection (page median), fix save/finish buttons

Bold detection:
- Replace absolute threshold with page-level relative comparison
- Measure stroke width for all cells, then mark cells >1.4× median as bold
- Adapts automatically to font, DPI and scan quality

Save buttons:
- Fix status stuck on 'error' preventing re-click
- Better error messages with response body
- Fallback score to 0 when null

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

admin-lehrer/components/ocr-pipeline/StepGroundTruth.tsx

@@ -170,22 +170,29 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
 
   // Save validation
   const handleSave = async () => {
-    if (!sessionId) return
+    if (!sessionId) {
+      setError('Keine Session-ID vorhanden')
+      return
+    }
     setStatus('saving')
+    setError('')
     try {
       const resp = await fetch(
         `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/reconstruction/validate`,
         {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
-          body: JSON.stringify({ notes, score }),
+          body: JSON.stringify({ notes, score: score ?? 0 }),
         }
       )
-      if (!resp.ok) throw new Error(`Save failed: ${resp.status}`)
+      if (!resp.ok) {
+        const body = await resp.text().catch(() => '')
+        throw new Error(`Speichern fehlgeschlagen (${resp.status}): ${body}`)
+      }
       setStatus('saved')
     } catch (e) {
       setError(e instanceof Error ? e.message : String(e))
-      setStatus('error')
+      setStatus('ready')
     }
   }