fix(plans): empty-step run-now follow-ups: drop the internal issue reference from the 409, make the disabled reason reachable #1708

Follow-up to #1695 / #1706, from review notes on #1706.

1. User-facing 409 text names an internal issue

When run-now is refused for a plan step with no recommendations, the API answers 409 with purchase.ErrPlanStepNoRecommendations text (internal/purchase/execution.go:29). That text ends with "(platform#631: plan steps cannot yet carry recommendations)". The retry refusal for the same case (internal/api/handler_purchases.go, retryPurchase) carries the same suffix. User-facing errors shouldn't reference internal issue numbers.

Expected: user-facing text such as "This plan step has no recommendations attached, so there is nothing to run." Keep any issue reference in logs or code comments only.

2. The disabled Run now button's reason may not be reachable

#1706 renders Run now disabled for an empty step, and puts the reason in title and aria-label. A disabled <button> isn't keyboard-focusable, and some browsers don't show title tooltips on disabled controls. So sighted keyboard users, and some pointer users, may never see why the button is disabled.

Expected: check it in Chrome, Firefox and Safari. If the reason isn't reachable, use aria-disabled="true" (focusable, with the click handled as a no-op), or show the reason as visible text in the row, for example next to "No recommendations attached".

3. Revisit when #631 lands

Once plan steps carry recommendations (#631), empty steps should become rare. Re-check whether the empty-step guards in runPlannedPurchase and retryPurchase, and the disabled-button state, are still needed, and keep their wording aligned.

Related

• fix(plans): Run now on a step with no recommendations promises a no-op but fails with an unexpected error #1695, fix(plans): refuse run-now on a step with no recommendations instead of a 500 #1706: the run-now fix.
• feat(plans): plan steps resolve real recommendations and show real counts and prices (tier B of #609) #631, fix(plans): a plan with no matching recommendations schedules zero-count purchases instead of failing #609: plan steps carrying recommendations.