fix(frontend): don't offer Exchange for reshape rows with no target #1410

On the RI-exchange reshape page, frontend/src/riexchange.ts ~985/990 renders a recommendation with no target as "0x " and still shows a live Exchange button. With the cloud-commitments-go bump in #1246 (go#584 offered instance types, and 0dd6bde capping reshape targets at owned units), more recommendations legitimately come back with no target, so this shows up more often.

Fix: render no-target rows with their Reason (or a clear "no exchange target available" message), disable or hide the Exchange button for them, and add a jest test. Found in an independent review of #1246.