fix: deduplicate overlapping OCR words and use per-word Y positions in overlay

Backend: Add spatial overlap check (>=50% horizontal IoU) to Kombi merge
so words at the same position are deduplicated even when OCR text differs.

Frontend: Add yPct/hPct to WordPosition so each word renders at its actual
vertical position instead of all words collapsing to the cell center Y.

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

admin-lehrer/components/ocr-overlay/OverlayReconstruction.tsx

@@ -513,9 +513,9 @@ export function OverlayReconstruction({ sessionId, onNext }: OverlayReconstructi
                         className="absolute leading-none pointer-events-none select-none"
                         style={{
                           left: `${wp.xPct}%`,
-                          top: `${bboxPct.y}%`,
+                          top: `${wp.yPct}%`,
                           width: `${wp.wPct}%`,
-                          height: `${bboxPct.h}%`,
+                          height: `${wp.hPct}%`,
                           fontSize: `${fs}px`,
                           fontWeight: globalBold ? 'bold' : 'normal',
                           fontFamily: "'Liberation Sans', Arial, sans-serif",
@@ -534,9 +534,9 @@ export function OverlayReconstruction({ sessionId, onNext }: OverlayReconstructi
                   return (
                     <div key={`${cell.cellId}_wp_${i}`} className="absolute group" style={{
                       left: `${wp.xPct}%`,
-                      top: `${bboxPct.y}%`,
+                      top: `${wp.yPct}%`,
                       width: `${wp.wPct}%`,
-                      height: `${bboxPct.h}%`,
+                      height: `${wp.hPct}%`,
                     }}>
                       <input
                         id={`cell-${cell.cellId}`}