fix library collections: remove duplicates, correct counts

- Delete 2 empty duplicate "Supplemental" collections from bib.sqlite
  (ERRVP346 and JK6N2CLM — 0 items each, leftover from Zotero sync)
- Fix export: item_count now shows direct items only (matches what the
  filter actually returns), not misleading recursive totals
- Drop collections with 0 direct items from export (they produce empty
  results when clicked)

Before: 19 collections, 3x "Supplemental", PFS showing 12429
After:  8 collections, 1x "Supplemental", PFS showing 254

docs/scripts/export_library.py

@@ -42,9 +42,14 @@ def main() -> None:
 
     store = Store(str(BIB_DB))
     items = store.list_items()
-    collections = store.list_collections()
+    raw_collections = store.list_collections()
     tags = store.list_tags()
 
+    # item_count from list_collections() is direct-only, which is what
+    # the library page filter uses (i.collections.includes(key)).
+    # Drop collections with 0 direct items — they produce empty results.
+    collections = [c for c in raw_collections if c["item_count"] > 0]
+
     serialized_items = []
     for item in items:
         abstract = item.abstract or ""