Add a third transfer mode that inserts new rows and updates existing
rows based on primary key, supporting PostgreSQL, MySQL, SQLite,
SQL Server, Oracle, and DuckDB with database-specific upsert syntax.
Also fixes DuckDB primary key detection in transfer operations.
Introduce src-web (axum HTTP server) exposing all 43 database operations
as REST APIs with SSE streaming for AI, transfers, and SQL file execution.
Frontend adapts automatically between Tauri and HTTP backends via api.ts.
Includes password authentication, Docker packaging, and login page.