feat: portal selector home page + remove self-serve signup

Replace the home page with a portal selector showing three access paths:
- Organisation Portal → /org/login
- Chapter Portal → /login
- Member Portal → /member-login

Remove /signup and its BFF route — tenants are now only created by org
owners through the org portal or assigned by super admins. Self-serve
community creation is no longer supported.

Update sidebar to bypass auth guard for / (exact match) without
accidentally matching all paths via startsWith.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/app/api/auth/signup/route.ts

@@ -1,19 +0,0 @@
-import { getApiBaseUrl, jsonResponse } from '../../../_lib/api';
-
-export const dynamic = 'force-dynamic';
-
-export async function POST(req: Request): Promise<Response> {
-  const body = await req.json();
-  const upstream = await fetch(`${getApiBaseUrl()}/auth/signup`, {
-    method: 'POST',
-    headers: { 'Content-Type': 'application/json', Accept: 'application/json' },
-    body: JSON.stringify(body),
-    cache: 'no-store',
-  });
-  const payload = await upstream.json();
-  if (!upstream.ok) return jsonResponse(payload, upstream.status);
-  const token: string | undefined = payload?.token;
-  if (!token) return jsonResponse({ message: 'Signup response missing token' }, 502);
-  const cookieValue = `tower_token=${token}; Path=/; HttpOnly; SameSite=Lax; Max-Age=${60 * 60 * 24 * 7}`;
-  return jsonResponse({ admin: payload.admin }, 200, { 'Set-Cookie': cookieValue });
-}