feat(content): add reusable partials and how-to template

src/content/partials/paid-addon-callout.mdx

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+<div class="anhh-accent-callout">
+  **Add-on, billed separately.** This feature is not included in the base hosting plan. You can enable and price it from your [client portal](https://secure.anhonesthost.com/clientarea.php).
+</div>

src/content/partials/service-hostnames.mdx

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+Some applications need to connect to databases, caches, or other internal services. Use these hostnames from inside your site — they only resolve within your hosting container, so they don't work from your laptop.
+
+| Service                | Hostname                | Default port | Notes                                                  |
+| ---------------------- | ----------------------- | ------------ | ------------------------------------------------------ |
+| MySQL                  | `mysql`                 | 3306         | Username, password, and database from your WHP panel.  |
+| PostgreSQL             | `postgres`              | 5432         | Available on most plans; ask support if it's missing.  |
+| Memcached              | `memcache`              | 11211        | No auth; isolated per container.                       |
+| Redis                  | `redis`                 | 6379         | Single DB per site; password in panel.                 |
+| SMTP (outbound)        | `smtp.anhonesthost.com` | 587          | Use your full email address as the username.           |
+
+**Connecting from your laptop?** Use the panel's **phpMyAdmin** (MySQL) or **Adminer** (PostgreSQL) for ad-hoc queries — direct external connections to these internal services are not exposed for security reasons.

src/content/partials/signing-in.mdx

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+import { URLS } from './urls.mdx';
+
+You can sign in to WHP two ways:
+
+1. **Through your client portal** (recommended). Go to [{URLS.whmcs}]({URLS.whmcs}), sign in to your account, open **Services → My Services**, click your hosting plan, then click **Login to WHP**. No extra password to remember.
+2. **Directly with your WHP credentials.** Visit `https://<your-server-hostname>:8443` and sign in with the WHP username and password you set up. Your server hostname is in your welcome email and on the service page in the client portal.

src/content/partials/support-link.mdx

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+import { URLS } from './urls.mdx';
+
+<div class="anhh-accent-callout">
+  **Still stuck?** Open a [support ticket]({URLS.whmcsTicket}) and our team will help.
+</div>

src/content/partials/urls.mdx

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+export const URLS = {
+  whmcs: 'https://secure.anhonesthost.com',
+  whmcsTicket: 'https://secure.anhonesthost.com/submitticket.php',
+  whmcsClientArea: 'https://secure.anhonesthost.com/clientarea.php',
+  publicSite: 'https://anhonesthost.com',
+  whpDirectExample: 'https://your-server-hostname:8443',
+};