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/ui/TextField.tsx

@@ -1,4 +1,4 @@
-import { forwardRef, type InputHTMLAttributes } from 'react'
+import { forwardRef, useId, type InputHTMLAttributes } from 'react'
 import { cn } from '@/lib/cn'
 
 interface TextFieldProps extends InputHTMLAttributes<HTMLInputElement> {
@@ -7,12 +7,14 @@ interface TextFieldProps extends InputHTMLAttributes<HTMLInputElement> {
 }
 
 // A labelled text input. text-base (16px) is deliberate: smaller fonts make iOS
-// Safari zoom on focus. The label's htmlFor falls back to the field name.
+// Safari zoom on focus. The field id falls back to the name, then to a generated
+// id, so the label/hint associations always hold.
 export const TextField = forwardRef<HTMLInputElement, TextFieldProps>(function TextField(
   { label, hint, id, name, className, ...props },
   ref,
 ) {
-  const inputId = id ?? name
+  const generatedId = useId()
+  const inputId = id ?? name ?? generatedId
   const hintId = hint ? `${inputId}-hint` : undefined
   return (
     <div className="flex flex-col gap-1.5">