Address Gadfly review on #6: logout errors, register gating, guard errors

Fixes from the PR #25 adversarial review (graded 23 real / 5 false positive):

Error handling
- onLogout wraps mutateAsync in try/catch: a failed logout no longer
  becomes an unhandled rejection; the user stays put (session still valid)
  and the button offers "Retry sign out" (5 models flagged this).
- Root errorComponent (RouteError): a non-401 /auth/me failure in a
  beforeLoad guard now shows a recoverable "Try again" screen instead of
  blanking.
- Forms drop noValidate, restoring native required/type=email/minLength
  checks before hitting the server.

Correctness
- RegisterPage gates on providers.isPending/isError before rendering, so
  the form no longer briefly appears (submittable) on an SSO-only server.
- TextField id falls back to name then a useId() value, so the label/hint
  associations hold even if a caller omits both.

Maintainability
- Single safeRedirectPath (lib/redirect.ts) replaces the duplicated
  safeRedirect/safeInternalPath (4 models).
- Generic ApiError helpers (apiErrorCode, errorMessage) moved to lib/api.ts;
  LoginPage builds the OIDC URL from the exported API_BASE.
- Divider moved to components/ui/. errorMessage doc clarified.

Verified again in a real browser: register -> /gardens; Sign out -> /login.
tsc --noEmit and vite build clean.

Not changed (graded, with rationale): OIDC deep-link redirect isn't
preserved (needs backend state threading — follow-up); 60s me staleTime in
guards (server is authoritative); the login/register onSubmit catches are
the intended react-query pattern (errors render via mutation state).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

web/src/components/RouteError.tsx

@@ -0,0 +1,17 @@
+import { useRouter } from '@tanstack/react-router'
+import { Button } from '@/components/ui/Button'
+import { errorMessage } from '@/lib/api'
+
+// Root error boundary: if a route's beforeLoad/loader throws something other
+// than a redirect (e.g. /auth/me fails with a network error or a 500), show a
+// recoverable message instead of a blank screen. Retrying re-runs the guards.
+export function RouteError({ error }: { error: Error }) {
+  const router = useRouter()
+  return (
+    <div className="mx-auto flex min-h-[60vh] w-full max-w-sm flex-col items-center justify-center gap-4 text-center">
+      <h1 className="text-lg font-semibold text-fg">Something went wrong</h1>
+      <p className="text-sm text-muted">{errorMessage(error, 'An unexpected error occurred.')}</p>
+      <Button onClick={() => router.invalidate()}>Try again</Button>
+    </div>
+  )
+}