Two related fixes for OAuth login failures:
- `UserSchema`: backend (Facebook/Apple) returns `null` for fields like
`email`; Zod's `.default("")` rejects `null`, causing silent login
failures. Introduce `stringOrEmpty` / `numberOrZero` / `booleanOrFalse`
helpers (`nullable().transform(v => v ?? x).default(x)`) and apply
across the schema so input accepts `string | null | undefined` while
output stays the typed primitive.
- `SplashScreen`: render `state.errorMessage` (set by auth state machine
`onError`, e.g. Facebook sync / schema mismatch) so users no longer
get stuck on splash without feedback.
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