Fix en/de mode edge case on docs without detected English column

When no IPA signals exist (e.g. German-only dicts), the fallback
that guesses en_col_type was incorrectly triggered for en/de modes,
causing false IPA and syllable insertions. Now only fires for 'all'
mode. Syllable en mode also returns empty set when no EN column found.

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

klausur-service/backend/grid_editor_api.py

@@ -889,8 +889,8 @@ async def _build_grid_core(
                 col_ipa_count[ct] = col_ipa_count.get(ct, 0) + 1
         if col_ipa_count:
             en_col_type = max(col_ipa_count, key=col_ipa_count.get)
-        elif ipa_mode in ("all", "de", "en"):
-            # Force mode without auto-detection: pick column with most cells
+        elif ipa_mode == "all":
+            # Force-all mode without auto-detection: pick column with most cells
             col_cell_count: Dict[str, int] = {}
             for cell in all_cells:
                 ct = cell.get("col_type", "")
@@ -1510,10 +1510,12 @@ async def _build_grid_core(
             _syllable_eligible = True
         # For language-specific modes, determine allowed columns
         _syllable_col_filter: Optional[set] = None  # None = all columns
-        if syllable_mode == "en" and en_col_type:
-            _syllable_col_filter = {en_col_type}
-        elif syllable_mode == "de" and en_col_type:
-            _syllable_col_filter = all_content_cols - {en_col_type} if total_cols >= 3 else None
+        if syllable_mode == "en":
+            _syllable_col_filter = {en_col_type} if en_col_type else set()
+        elif syllable_mode == "de":
+            if en_col_type and total_cols >= 3:
+                _syllable_col_filter = all_content_cols - {en_col_type}
+            # else None → all columns (correct for German-only dicts)
         if _syllable_eligible:
             try:
                 from cv_syllable_detect import insert_syllable_dividers