feat(app): redesign auth — pixel-art landing, login, register, verify screens

[MarkQuach12]

Summary

• Replaces Clerk's prebuilt auth UI with four custom pixel-art screens matching the approved mocks: landing (jematala wordmark + login/register + paw scatter), login (username + password), register (email / username / password / confirm), and verify (6-digit emailed code with auto-submit and 30s resend cooldown)
• One cross-platform implementation (native + web via RN Web): deletes the four platform-split auth files and lib/clerkAppearance.ts
• New shared pieces: authCream/authSage/authSageDark theme tokens, sage Button variant, AuthScreen/PawScatter chrome, PixelInput/FormError/clerkErrorMessage primitives, 32×32 paw sprite
• Clerk resource hooks import from @clerk/expo/legacy (v3.2.14 moved useSignIn/useSignUp there; verified to share the main ClerkProvider context)
• Signed-in users are bounced off all auth screens; signed-out flow: / landing → sign-in/sign-up → (verify) → map

Verification

• bun run check (oxlint + oxfmt + 4× tsc) passes; bun run build:app web export passes

Manual test checklist (before merge)

• Clerk dashboard: username attribute enabled (login is by username)
• Web: landing → register (throwaway email) → wrong code shows red error → resend shows sent! (30) countdown → real code auto-submits → map
• Browser/hardware back from map does NOT land on a usable register form (should bounce to map)
• Sign out → login with wrong password (red error line) → correct username+password → map
• Native: keyboard doesn't cover register's bottom field; paws render crisply; deep link to /verify while signed out bounces to landing