fix(login): one-button Google sign-in + middleware auto-issues dev cookie

The dev login flow was redirecting back to /login because:
  - src/middleware.ts didn't exist, so the Auth.js authorized
    callback in auth.config.ts never ran
  - Even if it had, it only checked the Auth.js JWT, not dev_session
  - Clicking the demo buttons set the cookie via document.cookie,
    but the admin layout (via getAdminUser) was the only thing
    reading it — no edge gate

Fix:
  - New src/middleware.ts: gates /admin/* and /login at the edge.
    Auto-issues dev_session=platform_admin when ALLOW_DEV_LOGIN is
    enabled (default on, set to 'false' in prod). No buttons, no
    client-side cookie games.
  - LoginClient.tsx: stripped to a single Google OAuth button.
    Removed email/password form, dev credentials form, and the
    /login?demo=1 three-button picker.
  - Removed signInWithDev from auth-signin.ts (no longer called).
  - Removed dead /dev-login page and /api/dev-login route.

Net result: one sign-in path (Google), invisible dev auto-login
via middleware, no more three modes.

src/app/api/dev-login/route.ts

@@ -1,21 +0,0 @@
-import { NextResponse } from "next/server";
-
-export async function POST() {
-  const response = NextResponse.redirect(new URL("/admin", "http://localhost:3000"));
-  response.cookies.set("dev_session", "platform_admin", {
-    path: "/",
-    sameSite: "lax",
-    httpOnly: false,
-  });
-  return response;
-}
-
-export async function GET() {
-  const response = NextResponse.redirect(new URL("/admin", "http://localhost:3000"));
-  response.cookies.set("dev_session", "platform_admin", {
-    path: "/",
-    sameSite: "lax",
-    httpOnly: false,
-  });
-  return response;
-}