Guide first-time Linq sign-in

[fmhall]

Summary

Desktop

Mobile

Make Linq's required first-time sign-in flow visible before a user requests an OTP.

• show the three required steps: open Messages, send any message from the sign-in phone, then return and request a code
• add a Text Linq in Messages button with an sms: deep link to the deployment's onboarding number
• prefer the optional LINQ_PHONE_NUMBER override and otherwise resolve the number through Linq's V3 available_number endpoint
• keep the instructions visible with a manual Vercel Connect/Linq dashboard fallback when number resolution is unavailable
• distinguish recipient verification (2008) from sender authorization (2006) and no-eligible-line failures

Behavior and boundaries

The button opens the user's Messages client but does not send anything. The user still sends the first message explicitly. The lookup is app-scoped, bounded to three seconds, validated as E.164, and fails soft so Linq or Connect availability cannot prevent the sign-in form from rendering.

Local OTP bypass and deployments without a Linq connector retain their existing behavior.

Validation

• pnpm exec vitest run src/auth/tests/linq.test.ts src/auth/tests/auth-linq.test.ts src/app/sign-in/_tests/phone-auth-form.test.ts — passed; 3 files and 19 tests
• pnpm check — passed; 46 test files and 220 tests, plus lint, TypeScript, formatting, Knip, and package boundaries
• pnpm exec turbo run build:app --force — passed after merging current main
• production-like local browser smoke test — rendered the ordered flow, confirmed sms:+12025550123, and served /sign-in with HTTP 200 from a clean Next cache
• independent review findings were remediated and the relevant checks rerun

Reviewer focus

• fail-soft behavior and latency of the unauthenticated onboarding-number lookup
• resolved-number, unresolved-number, unconfigured, and local-bypass render states
• separation between recipient onboarding, connector line authorization, and line-health errors

No changeset is needed for this application repository.