docs(white-labeling): document mail template placeholders

Add the available placeholders for each mail template and a section
describing what each one resolves to. Rename the template rows to match
the labels shown in the White labeling > Mail templates drop-down and
add the missing Test email message template.

Placeholders are only substituted in the mail body — the subject is sent
as plain text. Warn that a placeholder the selected template does not
provide breaks that template entirely: rendering fails, so no email of
that type is sent at all until the placeholder is removed.

Author: hophead12

src/content/_includes/docs/user-guide/white-labeling-mail.mdx

@@ -28,14 +28,40 @@ To send emails, a tenant administrator must first configure an <DocLink product=
 
 The following mail templates are available for customization:
 
-| Template | Triggered when |
-|----------|---------------|
-| **Activation** | A new user is created and receives an activation link |
-| **Account activated** | A user successfully activates their account |
-| **Account lockout** | A user's account is locked due to too many failed login attempts |
-| **Password reset** | A user requests a password reset |
-| **Password was reset** | A user's password has been successfully changed |
-| **Two-factor authentication verification** | A 2FA code is sent to the user |
+| Template | Triggered when | Available placeholders |
+|----------|----------------|------------------------|
+| **Test email message** | An administrator sends a test email while configuring the outgoing mail server | `${targetEmail}` |
+| **Account activation message** | A new user is created and receives an activation link | `${activationLink}`, `${activationLinkTtlInHours}`, `${targetEmail}` |
+| **Account activated message** | A user successfully activates their account | `${loginLink}`, `${targetEmail}` |
+| **Account lockout message** | A user's account is locked due to too many failed login attempts | `${lockoutAccount}`, `${maxFailedLoginAttempts}`, `${targetEmail}` |
+| **Reset password message** | A user requests a password reset | `${passwordResetLink}`, `${passwordResetLinkTtlInHours}`, `${targetEmail}` |
+| **Password was reset message** | A user's password has been successfully changed | `${loginLink}`, `${targetEmail}` |
+| **2FA verification message** | A 2FA code is sent to the user | `${code}`, `${expirationTimeSeconds}`, `${targetEmail}` |
+
+## Placeholders
+
+The **mail body** is processed as a template, so it supports placeholders written as `${placeholderName}`. The platform substitutes them with actual values before sending the email. The **mail subject** is sent as plain text — placeholders in the subject are not substituted.
+
+Use only the placeholders listed for the given template above.
+
+<Aside type="caution">
+A placeholder that the selected template does not provide breaks that template entirely. The message fails to render, so **no email of that type is sent at all** — for any user, and permanently, until you remove the placeholder. The platform does not fall back to the default template and does not leave the placeholder in the text as-is.
+
+For example, adding `${loginLink}` to the **Account activation message** template stops every account activation email, because that template provides only `${activationLink}`, `${activationLinkTtlInHours}` and `${targetEmail}`. The same applies to a typo — `${targetEmai}` breaks the template just as much as an unrelated name.
+</Aside>
+
+| Placeholder | Substituted with |
+|-------------|------------------|
+| `${targetEmail}` | Email address the message is sent to |
+| `${activationLink}` | One-time link the user opens to activate their account |
+| `${activationLinkTtlInHours}` | Lifetime of the activation link, in hours (rounded up) |
+| `${loginLink}` | Link to the platform login page |
+| `${lockoutAccount}` | Email address of the account that has been locked out |
+| `${maxFailedLoginAttempts}` | Number of failed login attempts that triggers the lockout |
+| `${passwordResetLink}` | One-time link the user opens to set a new password |
+| `${passwordResetLinkTtlInHours}` | Lifetime of the password reset link, in hours (rounded up) |
+| `${code}` | Two-factor authentication verification code |
+| `${expirationTimeSeconds}` | Lifetime of the 2FA verification code, in seconds |
 
 <Aside type="tip">
 Mail templates support HTML. You can include your company logo, custom colors, and branding to match your white-labeled UI.