fix: add Enter key to trigger active primary buttons

[patrickoleary]

Add global Enter key support that routes to the currently active primary action button.

• Global keydown listener registered via ClientTriggers — fires regardless of input focus
• Server-side _enter_action routes Enter based on active context:

1. State export dialog → triggers save/download
2. File loading dialog → imports state file if selected, otherwise loads simulation + connectivity files
3. Variable selection → loads selected variables

Each action respects the same disabled conditions as its button

closes #79

[jourdain]

Wrong solution, the various places where we expect "enter" to validate current action should handle it locally with the standard event binding of vue.

[patrickoleary]

No I tried that. It won't work because it doesn't have focus. so you can't do it locally.

[jourdain]

Make the field or button in focus

[jourdain]

And if you don't want to have anything in focus and just handle "enter", then you should use our current set of keyboard shortcut here