Because:
- Send Tab entrypoints need a "send some tabs" layout on /pair and /pair/auth/complete
- PR #20333 added three /pair success banners via query params, but the React conversion only renders one
This commit:
- getSyncNavigate branches on config.showReactApp.pairRoutes: React soft-navs with locationState.origin; Backbone keeps the hard-nav + query params
- Pair/Index renders all three banner variants above the headings; fixes pair-cad-header to "Connect another device"
- Adds a Send Tab variant to /pair (both apps) with a single H1 "Download or open Firefox on the device where you want to send tabs"
- Adds a Send Tab variant to /pair/auth/complete (both apps) with "You're ready to send some tabs" + dynamic device copy, no CTA
- Adds SEND_TAB_ENTRYPOINTS to fxa-content-server constants
closes FXA-13397
closes FXA-13414