fix: add Enter key to trigger active primary buttons

[patrickoleary]

New approach:
Uses local Vue @keyup.enter handlers on each dialog/panel to trigger the primary action on Enter/Return. Works regardless of which element has focus (text fields, checkboxes, table rows).

Panels handled:

• File loading dialog → loads files or imports state file
• State export dialog → saves/downloads state file
• Variable selection drawer → loads selected variables

Each handler respects the same disabled conditions as its corresponding button.

closes #79

[jourdain]

integrated into #87