feat: isolate all four portals with route groups + polished design system

Fix blank /org/login (and /admin/login): the guarded portal layout was
wrapping its own login route, rendering null when unauthenticated. Move
each portal's authed pages into a (authed)/(chapter) route group so login
pages live outside the guard.

Structure:
- app/(chapter)/* — chapter admin pages + guarded layout (was root-level)
- app/org/(authed)/* — org pages + guarded layout; /org/login now free
- app/admin/(authed)/* — admin pages + guarded layout; /admin/login free
- Root layout slimmed to providers only (no shared sidebar)
- Convert moved files' relative imports to @/app alias

Design system:
- PortalShell: shared sidebar shell (brand mark, themed active nav with
  accent bar, avatar dropdown user menu, loading skeletons)
- portal-theme.ts: per-portal theme tokens (violet/slate/blue/emerald)
- PortalLoginShell redesigned: two-column with gradient branding panel,
  dotted pattern, value-prop highlights, built-in back link
- New shadcn components: Avatar, DropdownMenu, Skeleton
- Move shared DraftCard to _components/draft-card.tsx (used by 2 portals)
- Portal selector: remove Super Admin card, icon tiles, hover lift
- All 4 login pages use react-hook-form + Zod + themed shell
- Member nav restored to full 11-section list with icons

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

apps/web/app/admin/login/page.tsx

@@ -9,7 +9,6 @@ import { PortalLoginShell } from '../../_components/portal-login-shell';
 import { Form, FormControl, FormField, FormItem, FormLabel, FormMessage } from '../../_components/ui/form';
 import { Input } from '../../_components/ui/input';
 import { Button } from '../../_components/ui/button';
-import Link from 'next/link';
 
 const schema = z.object({
   email: z.string().email('Enter a valid email'),
@@ -36,8 +35,8 @@ export default function SuperAdminLoginPage() {
     <PortalLoginShell
       title="Super Admin"
       subtitle="Platform-wide administration access."
-      accentClass="bg-slate-900"
-      footer={<Link href="/" className="hover:text-foreground transition-colors">← Back to portal selector</Link>}
+      theme="slate"
+      highlights={['Manage organisations & chapters', 'Provision and pair WhatsApp bots', 'Oversee the entire platform']}
     >
       <Form {...form}>
         <form onSubmit={form.handleSubmit(onSubmit)} className="space-y-4">