Fixed: billing-day preview and month-end anchor dates

Plans billed on a fixed calendar day had two related problems. A plan anchored to the 31st that renewed in a 28- or 30-day month would charge on the last day of that shorter month, and then stay stuck there in later months instead of returning to the 31st once the month was long enough again. Separately, the Charge on Specific Days preview in the plan editor could name the wrong day, show no dates at all, or, for a plan anchored to the 31st, fail to load the section altogether.

Both are now fixed. A renewal or skip date lands on the earlier of the plan's anchor day or the month's actual length, and returns to the anchor day itself as soon as a month is long enough to have it again, so a plan anchored to the 31st is never permanently pulled earlier by having passed through February.

The preview now shows real dates. For any monthly or weekly billing day, the editor lists the actual dates Shopify will charge a customer who subscribes today, including an example for a customer who signs up after your cutoff setting, measured directly against Shopify's own billing behavior rather than approximated.

The preview fix reaches any shop with a monthly or weekly billing-day plan; the month-end anchor fix is narrower, affecting plans anchored to a day near the end of the month. Both changes apply to future billing dates only, not any charge that already happened.