fix(ui): rebind economics panel toggle from E to N (#124 review R2)

E is claimed by InputMap "interact" action — InputMapper consumes
it before _unhandled_key_input. N is free, adjacent to M (star map),
reads as "Numbers" for the economics monitor.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/ui/implant/economics_panel.gd

@@ -257,7 +257,7 @@ func _rebuild_panel() -> void:
 	_panel.add_component(_placeholder_notice)
 
 	_panel.add_component(ImplantSeparator.new())
-	_panel.add_component(ImplantTextBlock.new("[ ] select system · E close"))
+	_panel.add_component(ImplantTextBlock.new("[ ] select system · N close"))
 
 
 func _current_node() -> Dictionary: