Align demo runbooks with current seeded-credentials login flow

[Copilot]

This issue was primarily stale-branch triage: both Copilot review branches are already merged from a content perspective, while the remaining actionable residue was outdated demo/login guidance that no longer matches main. This PR updates the operational docs to reflect the current seeded credentials flow instead of the removed demo-bypass path.

• Branch triage outcome

  • Confirmed copilot/equismile-review-issue and copilot/review-and-fix-issues have no unique changes left vs main
  • Confirmed test-push-capability is patch-equivalent to main
  • Assessed feature/demo-live-maps-override as obsolete in its current form: its relevant live-maps/demo assets already exist on main in newer form

• Demo login docs

  • Replaced stale references to “Continue as Demo Vet” and /api/demo/sign-in
  • Updated guidance to the current flow: choose a seeded persona card, enter EquiSmile2026!, land on /my-day
  • Clarified failure modes for missing seed data vs missing DEMO_MODE

• Preview / runbook alignment

  • Updated Vercel preview and client-demo instructions to describe the seeded credentials experience
  • Corrected login/landing-page language from dashboard-era behaviour to current My Day behaviour
  • Kept the service-worker troubleshooting note, but reframed it as a stale cached-login symptom rather than an active feature

• Integration setup docs

  • Removed usage examples that depended on the deleted demo sign-in endpoint
  • Replaced them with guidance to reuse a real authenticated session cookie from the current login flow

Example of the documentation correction:

- Tap "Continue as Demo Vet"
+ Tap the seeded staff persona card, enter `EquiSmile2026!`, and sign in