fix(web): let an org with no plan link itself to the org that pays - #1958

🟢 Confidence 9/10 · safe to merge
A two-line route exemption in the plan gate, pinned by a new plan-gate test; only the new link copy itself is untested.
_{no findings · tests partial −1 · risk medium}

Lets an org with no plan open Settings → Billing through the plan gate, and points both plan pickers at that page with a "billed through another organization" link. The exemption is narrow and tested; safe to merge.

• isAllowedWithoutPlan exempts Settings with tab=billing from the plan redirect
• billThroughAnotherOrgLink adds the billing-settings link under both plan pickers

_{124a68f · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}