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/_lib/sidebar.tsx

@@ -22,7 +22,7 @@ const SUPER_ADMIN_LINKS = [
   { href: '/admin/drafts', label: 'AI Drafts' },
 ];
 
-const PUBLIC_PATHS = ['/login', '/signup', '/onboard', '/member-login'];
+const PUBLIC_PATHS = ['/login', '/onboard', '/member-login'];
 const ADMIN_PATHS = ['/admin'];
 const MEMBER_PATHS = ['/my'];
 const ORG_PATHS = ['/org'];
@@ -43,6 +43,7 @@ export function Sidebar() {
 
   useEffect(() => {
     if (loading) return;
+    if (pathname === '/') return;
     if (PUBLIC_PATHS.some((p) => pathname.startsWith(p))) return;
     if (ADMIN_PATHS.some((p) => pathname.startsWith(p))) return;
     if (MEMBER_PATHS.some((p) => pathname.startsWith(p))) return;
@@ -52,7 +53,7 @@ export function Sidebar() {
     }
   }, [loading, admin, pathname, router]);
 
-  if (PUBLIC_PATHS.some((p) => pathname.startsWith(p))) {
+  if (pathname === '/' || PUBLIC_PATHS.some((p) => pathname.startsWith(p))) {
     return (
       <nav className="w-52 shrink-0 bg-white border-r border-gray-200 p-4">
         <span className="font-bold text-base">TOWER</span>