Add auth UI: login/register pages, OIDC button, route guard (#6)

Frontend for pansy auth, rendering whatever /auth/providers reports.

- lib/auth.ts: zod-validated User/Providers, a shared meQueryOptions
  (a 401 resolves to null, not an error) reused by useMe() and the router
  guard, useProviders(), and useLogin/useRegister/useLogout mutations that
  prime/clear the me cache (logout also drops non-auth cached data).
- lib/queryClient.ts: one QueryClient shared by the React tree and the
  router context so guard and components hit the same cache.
- router.tsx: createRootRouteWithContext with the queryClient; requireAuth
  (redirects to /login?redirect=<dest>) on gardens/plants/editor, and
  requireGuest on login/register (bounces authed users away). Login search
  params (redirect, error) validated as optional; redirect targets are
  restricted to same-origin paths.
- pages/LoginPage: OIDC button (label from providers) linking to
  /auth/oidc/login, "or" divider, local email/password form, and friendly
  messages for the OIDC callback ?error= codes. RegisterPage: email +
  display name + password (min 8), registration-closed and SSO-only
  handling; auto-login lands on /gardens.
- AppShell: auth-aware nav — shows the user's display name + Sign out when
  logged in, Sign in otherwise; nav links only when authed.
- ui/: TextField (16px text to avoid iOS zoom, autocomplete + a11y),
  Button (+ shared buttonClasses for the OIDC anchor), Alert; AuthCard.

Verified in a real browser against the embedded binary: register →
auto-login → /gardens; refresh stays in; Sign out → /login; a logged-out
/gardens/1 redirects to /login and returns after login; OIDC button renders
with the Authentik label when configured. tsc --noEmit clean.

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

web/src/components/auth/AuthCard.tsx

@@ -0,0 +1,22 @@
+import type { ReactNode } from 'react'
+
+/** Centered card layout shared by the login and register pages. */
+export function AuthCard({
+  title,
+  subtitle,
+  children,
+}: {
+  title: string
+  subtitle?: string
+  children: ReactNode
+}) {
+  return (
+    <div className="mx-auto flex min-h-[70vh] w-full max-w-sm flex-col justify-center">
+      <div className="rounded-xl border border-border bg-surface p-6 shadow-sm">
+        <h1 className="text-xl font-semibold tracking-tight text-fg">{title}</h1>
+        {subtitle && <p className="mt-1 text-sm text-muted">{subtitle}</p>}
+        <div className="mt-5">{children}</div>
+      </div>
+    </div>
+  )
+}