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/layout/AppShell.tsx

@@ -21,8 +21,14 @@ export function AppShell() {
   const user = me.data
 
   async function onLogout() {
-    await logout.mutateAsync()
-    await navigate({ to: '/login' })
+    try {
+      await logout.mutateAsync()
+      await navigate({ to: '/login' })
+    } catch {
+      // The logout request failed, so the session is still valid server-side:
+      // leave the user where they are (the button re-enables for a retry) rather
+      // than pretending they're signed out. logout.isError drives the title below.
+    }
   }
 
   return (
@@ -55,9 +61,10 @@ export function AppShell() {
                 type="button"
                 onClick={onLogout}
                 disabled={logout.isPending}
+                title={logout.isError ? 'Sign out failed — try again' : undefined}
                 className="rounded-md px-3 py-1.5 text-sm font-medium text-muted transition-colors hover:text-fg disabled:opacity-60"
               >
-                {logout.isPending ? 'Signing out…' : 'Sign out'}
+                {logout.isPending ? 'Signing out…' : logout.isError ? 'Retry sign out' : 'Sign out'}
               </button>
             </div>
           ) : (