feat(adminsite): session guards and the auth screens

Route-group layouts do the guarding. A customer session on /staff/* is
redirected to its own home rather than shown a refusal -- there is nothing to
tell them about. This is UX only: admin enforces the same boundary with
RequireStaff/RequireCustomer and answers 404 rather than 403 for another
account's data, which is the layer that actually matters.

Signup carries the honeypot the backend expects and reports "check your
email" rather than claiming an account exists, matching a backend that
creates nothing until the link is opened.

Buttons match site/'s .btn--solid and .btn--line, neutral border included.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

adminsite/components/Button.tsx

@@ -0,0 +1,25 @@
+import clsx from "clsx";
+
+type Props = React.ButtonHTMLAttributes<HTMLButtonElement> & { variant?: "solid" | "line" };
+
+/*
+ * Matches site/'s .btn--solid and .btn--line exactly, including the neutral
+ * border on the secondary variant. site/ does not have an accent-outlined
+ * button and this app should not invent one.
+ */
+export function Button({ variant = "solid", className, ...rest }: Props) {
+    return (
+        <button
+            {...rest}
+            className={clsx(
+                "inline-flex items-center gap-2 rounded border px-4 py-2.5 text-[0.94rem] font-semibold",
+                "transition-[filter,border-color] duration-150 hover:brightness-110",
+                variant === "solid"
+                    ? "border-accent bg-accent text-accent-ink"
+                    : "border-rule bg-panel text-ink hover:border-ink-3",
+                rest.disabled && "cursor-not-allowed border-rule bg-panel text-ink-3 hover:brightness-100",
+                className,
+            )}
+        />
+    );
+}