Menu Bar: Implement File -> New, Open, Save (SQL file support)

[ZhuchkaTriplesix]

The File -> New, Open..., and Save buttons in the top menu bar are currently non-functional stubs.

Proposed Solution:
Implement file operations for .sql files:

• New: Clear the active SQL editor text.
• Open...: Open a file selector (file_selector package) to read a .sql file into the editor.
• Save: Save the current editor buffer to a file (using file_selector save dialogs).

This will require passing the active editor TextEditingController or using a state notifier to communicate from the menu bar to the active workspace.