Fix: reprocess button works after session resume + apply merge logic

Two bugs fixed:
1. reprocessPages() failed silently after session resume because
   successfulPages was empty. Now derives pages from vocabulary
   source_page or selectedPages as fallback.

2. process-single-page endpoint built vocabulary entries WITHOUT
   applying merge logic (_merge_wrapped_rows, _merge_continuation_rows).
   Now applies full merge pipeline after vocabulary extraction.

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

studio-v2/app/vocab-worksheet/useVocabWorksheet.ts

@@ -760,11 +760,28 @@ export function useVocabWorksheet(): VocabWorksheetHook {
 
   // Reprocess all successful pages with new IPA/syllable modes
   const reprocessPages = (ipa: IpaMode, syllable: SyllableMode) => {
-    if (!session || successfulPages.length === 0) return
+    if (!session) return
+
+    // Determine pages to reprocess: use successfulPages if available,
+    // otherwise derive from vocabulary source_page or selectedPages
+    let pagesToReprocess: number[]
+    if (successfulPages.length > 0) {
+      pagesToReprocess = successfulPages.map(p => p - 1)
+    } else if (vocabulary.length > 0) {
+      // Derive from vocabulary entries' source_page (1-indexed → 0-indexed)
+      const pageSet = new Set(vocabulary.map(v => (v.source_page || 1) - 1))
+      pagesToReprocess = [...pageSet].sort((a, b) => a - b)
+    } else if (selectedPages.length > 0) {
+      pagesToReprocess = [...selectedPages]
+    } else {
+      // Fallback: try page 0
+      pagesToReprocess = [0]
+    }
+
+    if (pagesToReprocess.length === 0) return
 
     setIsExtracting(true)
     setExtractionStatus('Verarbeite mit neuen Einstellungen...')
-    const pagesToReprocess = successfulPages.map(p => p - 1)
     const API_BASE = getApiBase()
 
     ;(async () => {