Fix 4 Grid Editor bugs: syllable modes, heading detection, word gaps

1. Syllable "Original" (auto) mode: only normalize cells that already
   have | from OCR — don't add new syllable marks via pyphen to words
   without printed dividers on the original scan.

2. Syllable "Aus" (none) mode: strip residual | chars from OCR text
   so cells display clean (e.g. "Zel|le" → "Zelle").

3. Heading detection: add text length guard in single-cell heuristic —
   words > 4 alpha chars starting lowercase (like "zentral") are regular
   vocabulary, not section headings.

4. Word-gap merge: new merge_word_gaps_in_zones() step with relaxed
   threshold (6 chars) fixes OCR splits like "zerknit tert" → "zerknittert".

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

klausur-service/backend/grid_editor_helpers.py

@@ -912,6 +912,13 @@ def _detect_heading_rows_by_single_cell(
             _REAL_IPA_CHARS = set("ˈˌəɪɛɒʊʌæɑɔʃʒθðŋ")
             if _text_has_garbled_ipa(text) and not any(c in _REAL_IPA_CHARS for c in text):
                 continue
+            # Guard: dictionary section headings are short (1-4 alpha chars
+            # like "A", "Ab", "Zi", "Sch").  Longer text that starts
+            # lowercase is a regular vocabulary word (e.g. "zentral") that
+            # happens to appear alone in its row.
+            alpha_only = re.sub(r'[^a-zA-ZäöüÄÖÜßẞ]', '', text)
+            if len(alpha_only) > 4 and text[0].islower():
+                continue
             heading_row_indices.append(ri)
 
         # Guard: if >25% of eligible rows would become headings, the