feat(billing): subscriptions view 2-col layout — drop dup bar, CTA to /pricing#units (#4141)

- 2-column grid (left md=5: plan + meter, right md=7: extras + ledger)
- Drop BillingUsageBarComponent from subs view — single meter bar via
  BillingMeterProgressComponent (T4 % display) remains
- Replace @click extrasCheckoutDialog modal trigger with v-btn to="/pricing#units"
  (feedback #7: single source of truth for pack pricing on /pricing)
- Add billing.subscriptions.extras.buyExtra i18n key (en + fr)
- Rename breakdown card CSS class to avoid meter-card count collision
- Rename extras card CSS class (billing-subscriptions__extras-card)
- Tests: update 3 obsolete assertions + add 5 new T6 assertions (82 total pass)

Author: PierreBrisorgueil