Billing/Plans UI polish issue fixes

Found while visually checking Task 8340 (Billing/Plans → list + tabbed details) on the live stack. Each item follows docs/standards/ui-style-guide.md. 1. Products › Plans: the plan list and the Plans knob cards (Default plan, Features…) share one tab. The knobs scroll under the list. Give the knobs their own section or tab so the tab is one list. 2. Products › Plans list header: "SELF-SERVE" truncates to "SELF-SER…" at 1440px. Shorten the header (for example "Self") or widen the slot. 3. Products › Plans › Default plan select truncates the plan name ("Business tr") at 390px. Let it take the row's full width on a phone. 4. Plan page Price tab: a free plan seeded as per-agent with min = max = 1 shows "Priced per agent" checked, which reads as a contradiction. Either hide per-agent controls on a plan with no price, or fix the seed. 5. Customers › Billing still edits billing details and plan changes in dialogs (BillingDetailsDialog, Change plan). The style guide says a record is edited in place. Move Details to inline edit on its sub-tab, as the tenant's /org/billing/details does. 6. /org/billing/details with no Stripe connection shows only "Billing isn't set up yet" under a bare breadcrumb, with no title. Give it the page title "Billing details", like the other detail pages. 7. Tenant Subscription page: the end date ("Ends Nov 3") sits beside the title, while the Products row says "Current plan · Ends Nov 3 · Free". Use the same state words on both. Out of scope: any API or billing-logic change, Stripe flows, a new page archetype. Done means: - Each item fixed (or rejected with a one-line reason in the PR body). - Affected pages checked at 390px and 1440px with screenshots. - `./dc check quick` and the changed-area vitest suites green.

All 7 Billing/Plans polish items fixed (web only); PR #6445 merged to develop as 34e5b2a by the lane. Quick gate + changed-area vitest green; visual check at 390/1440 done. Deploy superseded by a newer push. Follow-up draft #8396. Memory: used mem-18304.