feat: price equivalents in breakeven calculator

[rando128]

Description

Small improvements on the grid calculator introduced in PR #554:

• The equivalent market prices are now shown both for the rebound and buy trigger percentages
• The calculator is now accessible as long as there is a first buy grid executed. None valid recommendations are rendered as N/A

Related Issue

The first enhancement is in response to @peweb request

How Has This Been Tested?

This is working on my production server.

Screenshots (if appropriate):

Equivalent prices:

N/A indicator:

[chrisleekr]

Thank you @rando128 for the change!