refactor(provider): return results from every useAuth helper

[Bccorb]

What

Unify useAuth() on the same result convention as the headless client. Every provider helper returns SeamlessAuthResult and none of them throw.

Closes #92.

The problem was bigger than the issue described

#92 framed this as one split: client returns results, provider throws. Taking inventory showed useAuth() alone had four styles side by side:

Style	Helpers
Throws	deleteUser, updateCredential, deleteCredential, switchOrganization, listOAuthProviders, startOAuthLogin, finishOAuthLogin
Returns a result	login, verifyStepUpWithPasskey, verifyStepUpWithPasskeyPrf, verifyStepUpWithTotp
Returns a boolean	handlePasskeyLogin
Returns a null sentinel	refreshStepUpStatus

So there was no rule to learn, only per-method memorization, and #64 would have propagated that to every adapter.

Three silent breakages this could have shipped

The conversion compiles cleanly whether or not callers are updated, because a result object is always truthy. That makes the failure mode silent, and three call sites were affected:

• Login: if (passkeyResult) was always true, so a failed passkey login would have navigated home as if it succeeded
• PassKeyLogin: same shape, same outcome
• OAuthCallback: used .then() / .catch(), and since the helper no longer rejects, .then() ran on failure too and navigated home

All three now check error. Worth flagging because typecheck cannot catch this class, so any adopter doing the same migration should grep their own truthiness checks rather than trusting the compiler.

State updates still gated on success

Helpers that mutate provider state apply the change only when the call succeeds. switchOrganization still refreshes the session, deleteCredential still prunes local state, and logout keeps its finally so local state clears even on a failed sign-out.

Changes

• src/AuthProvider.tsx: every helper returns a result; AuthContextType updated
• src/views/{Login,PassKeyLogin,OAuthCallback}.tsx and src/components/OAuthProviderButtons.tsx: consume results
• README: the "two error styles" section is replaced with "one error style everywhere", and the useAuth() signature block and credential example are updated
• Tests updated; suite unchanged at 221 passing

Checks

• npm run typecheck, npm run lint, npm run format:check clean
• Full suite: 221 passed
• Changeset (minor) with a per-helper migration list