fix(fitbit): correct the setup guide to the classic Fitbit Web API

The Fitbit integration connects through the classic Fitbit Web API, so a
self-hoster registers a Fitbit developer app at dev.fitbit.com — but the
setup guide walked users through creating a Google Cloud OAuth client,
whose client id the Fitbit sign-in rejects (unauthorized_client). Rewrite
the guide for the dev.fitbit.com flow (app type, HTTPS callback, scopes,
PKCE), leave the old page as a pointer, and add a test that keeps the
documented scopes and endpoints in sync with the client so the guide
cannot drift from the code again. Flags the September 2026 sunset of the
classic Fitbit Web API.

src/components/settings/integrations/fitbit-card.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-// v1.12.0 — Google Health (Fitbit & Pixel) card. Mirrors the WHOOP card: a
+// v1.12.0 — Fitbit (Fitbit & Pixel) card. Mirrors the WHOOP card: a
 // BYO-key credentials form first, then an OAuth connect, then the
 // sync/test/disconnect action row + parked-resume banner. v1.12.1 — status
 // reads off the consolidated /api/integrations/status envelope (the per-card