Fix resumable signup on mobile

[raymondjacobson]

Summary

• show a retryable failure state when native account creation fails
• resume Identity-only accounts at handle selection and indexed incomplete accounts at profile completion
• preserve guest password recovery without advancing when the user edits their handle
• use the exported common route namespace so the shared web helper typechecks in mobile

Follow-up to #14530.

Test plan

• Mobile TypeScript check
• Web TypeScript check
• Mobile focused Jest tests (7 tests)
• Web sign-on route Vitest tests (4 tests)
• ESLint on all touched files

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Latest commit: a535b98

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