fix(linux): keep imported ids and empty strings, batch CSV import, XLSX

[datlechin]

• build_insert writes every given value except generated columns, so an imported id is kept. A generated column, a PostgreSQL GENERATED ALWAYS identity or a SQL Server IDENTITY column can't be mapped; the dialog greys it out with "The database fills this column". SQL Server's IDENTITY_INSERT is session state on a pooled connection, so it is refused rather than toggled.
• An empty CSV field stays an empty string instead of taking the column default.
• XLSX: an unreadable value fails the export naming the column; integers and decimals over 15 significant digits are written as text; a cell over 32,767 characters fails with a hint to use CSV; fractions of a second show; dates outside 1900 to 9999 are text; a sheet named History becomes History_.
• SQL export: schema-qualified table, generated columns left out, identity values kept (as Copy As INSERT).
• TSV and CSV copies no longer add a leading apostrophe.
• Show Row as JSON: an unreadable value shows an alert naming the column instead of null.
• CSV import parses and plans on the blocking pool, and sends multi-row VALUES statements under each engine's limits (SQL Server 1,000 rows and 2,000 parameters; ClickHouse 1,000 parameters and 8 KiB of values, since its parameters travel in the URL). Each statement adds 5 s to the save deadline. Failures name file lines; on a table without transactions the message says part may have landed.
• ImportLedger holds the process, inhibits logout and suspend, and asks "Stop Imports?" on close.
• A record over 4,096 fields is refused before padding; problems show inside the import dialog; Import is off when every column is Skip or the file has no rows.
• XML: CR, and tab and newline in names, are written as character references; a control character fails the export naming the column.
• PostgreSQL text: BC dates print as 0001-01-01 BC, offset seconds are kept.
• Docker tests on PostgreSQL, MySQL, ClickHouse and SQL Server for identity columns and batched import, including rollback on a failing second statement.
• Tests updated on purpose: the one that pinned skipping auto-increment values, 16-digit XLSX numbers, and the clipboard apostrophe expectations.
• Skipped: XLSX empty string vs NULL; rust_xlsxwriter drops empty strings and the only alternative is a ="" formula.