[Improved, Fixed] Billing checkout hotfix

Improvements

• The plan picker now opens the checkout modal with your selected tier and cadence already applied, eliminating the need to choose your plan twice.

Fixes

• Restored the direct route from the “See plans” and “Plan & billing” pages to checkout, so every organization can once again purchase or change a subscription.
• Fixed an onboarding edge case that blocked fully-onboarded organizations from reaching the plan picker.
• Ensured legacy Flex customers are still taken through the correct Flex-specific checkout flow, with boosts ending as expected.
• Corrected a fallback path so organizations without any jobs carry their chosen plan into onboarding instead of losing the selection.