## Because
- `showReactApp.pairRoutes` is on in stage and prod. React serves all pairing, CAD, and CAD-via-QR routes, so no user can reach the Backbone code for these routes.
- This PR is part of FXA-6117, "Decommission Backbone".
## This pull request
- Deletes the Backbone `pair/*`, `connect_another_device`, and `post_verify/cad_qr` views, templates, mixins, models, and specs.
- Removes the `pair/*` and `cad_qr` routes from `router.js`. `pair` and `connect_another_device` stay as React-only handlers, because other Backbone views still navigate there.
- Removes the `/oauth?channel_id` Backbone bypass in `add-routes.js`. The `pairRoutes` group is now always on.
- Moves `PAIRING_ROUTES` into `FRONTEND_ROUTES` and deletes `get-frontend-pairing.js` and `getFrontEndPairingRouteDefinition`.
- Removes the `pairRoutes` and `postVerifyCADViaQRRoutes` flags from content-server and fxa-settings config.
- Changes `getSyncNavigate` in fxa-settings `Signin/utils.ts` to always soft-navigate to `/pair` with `{ origin, pairReason }` location state.
- Deletes the `pairingFlowBackbone*.spec.ts` functional specs and the `isPairRoutesReact` gate, so the React pairing specs no longer skip.
- Updates fxa-settings tests that still expected the old `/pair?showSuccessMessage=true&pairReason=...` hard navigation. They now expect the soft navigation to `/pair` with `{ origin, pairReason }` state. This fixes the CI job "Integration Test - Frontends".
## Issue that this pull request solves
Closes: FXA-14376 (parent: FXA-6117)