Implement a workflow: Customer signup

Steps (in order):

1. Account — route /auth/signup: Name*, Email*, Password*
   (Zod: valid email, password >= 8). Primary "Create account".
2. Verify — route /auth/verify: 6-digit code + "Resend". On success -> step 3.
3. Company — route /onboarding: full-page RecordForm, fields
   Company name*, Domain, Country, Size. Primary "Finish".
4. Done — redirect to /dashboard with a welcome toast.
   Entry point: "Sign up" link on /auth/signin.
   Success end state: a Customer + Organization record created; land on /dashboard.
   Back / cancel: Back returns to the previous step keeping input;
   Cancel returns to /auth/signin.
   Errors: inline field errors; a failed verify keeps the code screen.
   Data model: Customer { name, email }, Organization { name, domain,
   country, size }.
   Building blocks: AuthCard for steps 1-2 (copy app/_components/auth.tsx),
   RecordForm formMode="page" for step 3.

Test scenarios (happy / unhappy):
TC-1 Valid signup through all 3 steps -> Customer + Organization created, land on /dashboard
TC-2 Step 1 invalid email / short password -> inline errors, Next blocked
TC-3 Step 2 wrong verify code -> error, stays on code screen
TC-4 Step 3 missing Company name -> inline error, Finish blocked
TC-5 Back from step 2 -> step 1 -> account input preserved
TC-6 Cancel at any step -> returns to /auth/signin, nothing created

Done when: every step has loading/error/success, Zod validation, the scenarios
above pass, tokens, light + dark, a11y, lint + types + build pass.