sign-in.tsx only ever covered login (per its own comment "sign-up screens
are follow-up work"). Adds the other half: email/password sign-up using
Clerk's Futures API (signUp.password() -> verifications.sendEmailCode()/
verifyEmailCode() -> finalize()), mirroring sign-in.tsx's two-step
credentials/verify shape. No invite-code step, since INVITE_REQUIRED is
globally off on the backend since launch.
Verified live on the Android emulator: real Clerk validation surfaces
correctly (breached-password rejection), and a valid submission correctly
sends a verification email and transitions to the code-entry step.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
adminandClaude Sonnet 5
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