Standalone SQL editor (back)

Two host-built CodeMirror editors mounted outside the data table, each one assembled by hand from createSqlExtensions() + buildCompletionContext(). Both editors share the loaded table's live schema via actions.getCompletionContext() and refresh whenever a derived column is added. Type Ctrl/Cmd+Space to see column names (with their DuckDB types) and DuckDB functions (with category + description) in the dropdown.