feat(auth): host GoTrue confirmation email template as static asset

GoTrue v2.186 only loads MAILER_TEMPLATES_* over HTTP(S) (local file/file://
paths are rejected), so serve the token_hash confirmation template from the
app's public dir for GoTrue to fetch and cache.

public/email-templates/confirmation.html

@@ -0,0 +1,4 @@
+<h2>Confirm your signup</h2>
+<p>Welcome to LinumIQ Tunnels. Click the link below to confirm your email address and activate your account:</p>
+<p><a href="{{ .SiteURL }}/auth/confirm?token_hash={{ .TokenHash }}&type=signup">Confirm your email</a></p>
+<p>If you did not create this account, you can safely ignore this email.</p>