Add transactional email system via SMTP

Introduce internal/email package with SMTP sending, embedded HTML/text
templates, and multipart MIME assembly. Emails use a generic EmailData
struct (recipient name, message, optional button, optional closing) so
new email types can be added without code changes.

Wired into signup (welcome email), team creation, and team member
addition. No-op mailer when SMTP_HOST is not configured.

internal/email/templates/base.txt

@@ -0,0 +1,13 @@
+{{if .RecipientName}}Hello {{.RecipientName}},
+
+{{end}}{{.Message}}
+{{if .Button}}
+
+{{.Button.Text}}: {{.Button.URL}}
+{{end}}{{if .Closing}}
+
+{{.Closing}}
+{{end}}
+
+---
+This is a transactional email from Wrenn (https://wrenn.dev).