Menu Bar: Implement Connection -> Read-only mode

[ZhuchkaTriplesix]

The Read-only button in the Connection menu does not toggle read-only states.

Proposed Solution:

• Toggle a isReadOnly state in the active connection workspace settings.
• For SQLite, acquire the connection with SqliteSessionMode.readOnly.
• For PostgreSQL/MySQL, run SET TRANSACTION READ ONLY; or block mutating SQL queries on the client side.
• Show a lock icon 🔒 in the workspace status/toolbar when read-only is active.