fix(login): back-button nav, single OTP channel, register back buttons

- Login: identify → password/otp now push() history (was replace()), so Back
  returns to the email screen instead of leaving to /register.
- Login: in Shell mode the OTP screen no longer shows an email/SMS toggle —
  the channel is fixed by how you signed in, so email sign-in never shows a
  stray SMS option.
- Login: hide phone (SMS) sign-in while OTP is mocked — a faked login code
  can't mint a session (AuthGate would bounce), so it can't work without SMS.
  Email OTP + password + Google stay real and working.
- Register: add Back buttons (email screen → sign in; profile screen → email).
- Share MOCK_OTP/DEMO_OTP via @/lib/otp.

src/lib/otp.ts

@@ -0,0 +1,16 @@
+/**
+ * Demo OTP fallback, shared by the login and registration flows.
+ *
+ * While the Twilio SMS sender is down we can't deliver real one-time codes. When
+ * MOCK_OTP is on, phone verification steps skip Supabase/Twilio and accept a fixed
+ * DEMO_OTP instead. Flip NEXT_PUBLIC_MOCK_OTP to "false" (or remove it) to restore
+ * real SMS — no other change needed.
+ *
+ * IMPORTANT: this only substitutes for a *secondary* verification (e.g. confirming a
+ * phone during registration/onboarding, where the session already exists). It cannot
+ * mock a *login* whose sole credential is the OTP — there the OTP verification is what
+ * mints the session, and a faked code produces no session (the dashboard's AuthGate
+ * would bounce the user straight back). Passwordless login therefore stays real.
+ */
+export const MOCK_OTP = process.env.NEXT_PUBLIC_MOCK_OTP === "true";
+export const DEMO_OTP = "123456";