Bug: deleting a table's header row corrupts the table on save/reload #15

Summary

• Deleting the header row of a table silently corrupts the table's structure across save/reload cycles.

Details

Reproduction:

1. Insert a table (default 2x2 with header row)
2. Delete the header row using the new "delete row" toolbar button (cursor in header)
3. Note the entry, switch days, come back

What happens: the serializer writes | | | + separator + body rows. On reload, markdown-it/GFM semantics make the first body row the new header — so a table saved with 1 body row becomes an empty header-only table (| | | + | --- | --- |), and its data row is reinterpreted. Verified with a test: after deleting the header row, the serialized markdown re-loads with the first row's cells as tableHeader.

Expected: either the serializer should not emit a header-less table without a marker, or deleting the header row should convert the next row to a header — either way the round trip must be lossless.

Root cause: serializeTable in src/markdown.ts always writes a | --- | separator after the first row regardless of whether that row is actually a header, and GFM parsing always treats the first row as a header when a separator follows.