fix(columns): filter phantom narrow columns + rename step to OCR-Zeichenkorrektur

Phantom column fix:
Adjacent tiny gaps (e.g. 11px + 35px) can create very narrow columns
(< 3% of content width) with 0 words. These are scan artefacts, not
real columns. New Step 9 in detect_column_geometry():
- Filter columns where width < max(20px, 3% content_w) AND words < 3
- After filtering, extend each remaining column to close the gap with
  its right neighbor, and re-assign words to correct column

Example from logs: 5 columns → 4 columns (phantom at x=710, width=36px
eliminated; neighbors expanded to cover the gap)

UI rename:
- 'Schritt 6: LLM-Korrektur' → 'Schritt 6: OCR-Zeichenkorrektur'
- 'LLM-Korrektur starten' → 'Zeichenkorrektur starten'
- Error message updated accordingly
(No LLM involved anymore — spell-checker is the active engine)

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

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

@@ -342,7 +342,7 @@ export function StepLlmReview({ sessionId, onNext }: StepLlmReviewProps) {
     return (
       <div className="flex flex-col items-center justify-center py-12 text-center">
         <div className="text-5xl mb-4">⚠️</div>
-        <h3 className="text-lg font-medium text-red-600 dark:text-red-400 mb-2">Fehler bei LLM-Korrektur</h3>
+        <h3 className="text-lg font-medium text-red-600 dark:text-red-400 mb-2">Fehler bei OCR-Zeichenkorrektur</h3>
         <p className="text-sm text-gray-500 dark:text-gray-400 max-w-lg mb-4">{error}</p>
         <div className="flex gap-3">
           <button onClick={() => { setError(''); loadSessionData() }}
@@ -387,7 +387,7 @@ export function StepLlmReview({ sessionId, onNext }: StepLlmReviewProps) {
       <div className="flex items-center justify-between">
         <div>
           <h3 className="text-base font-medium text-gray-700 dark:text-gray-300">
-            Schritt 6: LLM-Korrektur
+            Schritt 6: OCR-Zeichenkorrektur
           </h3>
           <p className="text-xs text-gray-400 mt-0.5">
             {status === 'ready' && `${vocabEntries.length} Eintraege bereit zur Pruefung`}
@@ -405,7 +405,7 @@ export function StepLlmReview({ sessionId, onNext }: StepLlmReviewProps) {
           {status === 'ready' && (
             <button onClick={runReview}
               className="px-5 py-2 bg-teal-600 text-white rounded-lg hover:bg-teal-700 transition-colors text-sm font-medium">
-              LLM-Korrektur starten
+              Zeichenkorrektur starten
             </button>
           )}
           {status === 'running' && (