Add A/B testing toggles for OCR quality steps

Each quality improvement step can now be toggled independently:
- CLAHE checkbox (Step 3: image enhancement on/off)
- MaxCols dropdown (Step 2: 0=unlimited, 2-5)
- MinConf dropdown (Step 1: auto/20/30/40/50/60)

Backend: Query params enhance, max_cols, min_conf on process-single-page.
Response includes active_steps dict showing which steps are enabled.
Frontend: Toggle controls in VocabularyTab above the table.

This allows empirical A/B testing of each step on the same scan.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/vocab-worksheet/components/VocabularyTab.tsx

@@ -120,6 +120,36 @@ export function VocabularyTab({ h }: { h: VocabWorksheetHook }) {
           </div>
         </div>
 
+        {/* OCR Quality Steps (A/B Testing) */}
+        <div className={`flex items-center gap-3 mb-3 flex-shrink-0 flex-wrap ${isDark ? 'text-white/60' : 'text-slate-500'}`}>
+          <span className="text-xs font-medium">Steps:</span>
+          <label className="flex items-center gap-1 text-xs cursor-pointer">
+            <input type="checkbox" checked={h.ocrEnhance} onChange={(e) => h.setOcrEnhance(e.target.checked)} className="rounded" />
+            CLAHE
+          </label>
+          <label className="flex items-center gap-1 text-xs">
+            <span>MaxCols:</span>
+            <select value={h.ocrMaxCols} onChange={(e) => h.setOcrMaxCols(Number(e.target.value))} className={`px-1 py-0.5 text-xs rounded border ${isDark ? 'border-white/20 bg-white/10 text-white' : 'border-gray-200 bg-white text-gray-600'}`}>
+              <option value={0}>unbegrenzt</option>
+              <option value={2}>2</option>
+              <option value={3}>3</option>
+              <option value={4}>4</option>
+              <option value={5}>5</option>
+            </select>
+          </label>
+          <label className="flex items-center gap-1 text-xs">
+            <span>MinConf:</span>
+            <select value={h.ocrMinConf} onChange={(e) => h.setOcrMinConf(Number(e.target.value))} className={`px-1 py-0.5 text-xs rounded border ${isDark ? 'border-white/20 bg-white/10 text-white' : 'border-gray-200 bg-white text-gray-600'}`}>
+              <option value={0}>auto</option>
+              <option value={20}>20</option>
+              <option value={30}>30</option>
+              <option value={40}>40</option>
+              <option value={50}>50</option>
+              <option value={60}>60</option>
+            </select>
+          </label>
+        </div>
+
         {/* Error messages for failed pages */}
         {h.processingErrors.length > 0 && (
           <div className={`rounded-xl p-3 mb-3 flex-shrink-0 ${isDark ? 'bg-orange-500/20 text-orange-200 border border-orange-500/30' : 'bg-orange-100 text-orange-700 border border-orange-200'}`}>