feat: dedicated ports per panel tier (8880/8881/8882)

Each panel now has its own dedicated port and is fully self-contained:
- Port 8880: User panel (end-user hosting dashboard)
- Port 8881: Reseller panel (account/package management)
- Port 8882: Admin panel (datacenter/server manager)

Changes:
- install.sh: PORT_USER/PORT_RESELLER/PORT_ADMIN constants; three separate
  nginx/Apache vhosts; UFW opens all three ports; Fail2Ban jail per port;
  credentials file shows all three URLs
- config.ini: stores port_user/port_reseller/port_admin
- Core.php: defines PORT_USER/RESELLER/ADMIN, detects CURRENT_PORTAL from
  SERVER_PORT so the API knows which tier is being accessed
- Auth.php: portalUrl() maps role → correct port for cross-portal redirects
- auth.php endpoint: returns portal_url on login so JS redirects to right port
- index.php login: uses portal_url from API response (no hardcoded paths)
- admin/index.php: inline login form (port 8882 is self-contained, no redirect)
- user/index.php: inline login form (port 8880 self-contained)
- reseller/index.php: new full reseller panel with inline login (port 8881);
  sidebar with accounts, packages, DNS, branding, bandwidth report sections

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

panel/api/endpoints/auth.php

@@ -13,8 +13,9 @@ match ($action) {
         $user = $auth->user();
         audit('login', 'auth');
         Response::success([
-            'token' => $token,
-            'user'  => [
+            'token'       => $token,
+            'portal_url'  => Auth::portalUrl($user['role']),
+            'user'        => [
                 'id'       => $user['id'],
                 'username' => $user['username'],
                 'email'    => $user['email'],