decision: is the Read-only connection mode a guarantee or a hint? #1773

For PostgreSQL and MySQL, Read-only in the SQL editor sets `default_transaction_read_only = ON` (resp. the MySQL session transaction mode) on a pooled session. It is a default, not an enforcement: a script can run `BEGIN READ WRITE` or `SET default_transaction_read_only = off` and then write, and the pool slot stays writable for later tabs. SQLite opens the file read-only, so it is a guarantee there; MCP has its own statement guard.

Decide which it is:

1. A hint: say so in the UI text and the docs.
2. A guarantee: refuse mutating statements in the editor while the toggle is on (`containsMutatingSql` exists), including `SET ... read_only`, `BEGIN READ WRITE` and `SET TRANSACTION READ WRITE`, and reset the slot's setting when it is taken from the pool.

Found in the review of the SQL safety layer.