Add GT button to OCR overlay, prominent category picker, track pipeline

- Ground Truth button on last step of Pipeline/Kombi modes in ocr-overlay
- Prominent category picker in active session info bar (pulses when unset)
- GT badge shown when session has ground truth reference
- Backend: auto-detect pipeline from ocr_engine, store in GT snapshot
- Pipeline info shown in GT session list and regression reports
- Also pass pipeline param from ocr-pipeline StepGroundTruth

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

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

@@ -210,7 +210,7 @@ export function StepGroundTruth({ sessionId, onNext }: StepGroundTruthProps) {
     setGtMessage('')
     try {
       const resp = await fetch(
-        `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/mark-ground-truth`,
+        `${KLAUSUR_API}/api/v1/ocr-pipeline/sessions/${sessionId}/mark-ground-truth?pipeline=ocr-pipeline`,
         { method: 'POST' }
       )
       if (!resp.ok) {