email: change font, template common parts

Using the newer Jellyfin logo font for the header and hanken grotesk for the body.
Tried to redo emails with maizzle because using tailwind sounded nice, but getting it
to look like a17t would be more trouble than it's worth, since you can't
use CSS vars in emails and a17t uses them heavily. Instead, cleaned up
the mj-header a little and stored it in a separate file, and also the
header & footer, and changed the template vars with {{ .header }}  and
{{ .footer }} for all emails. Values are determined by
CustomContentInfo.Header/FooterText funcs. nil values are replaced at
program start by _runtimeValidator.

also, i beg of you don't try to do light/dark mode with mjml, you'll
want to die.

customcontent.go

@@ -19,6 +19,18 @@ func defaultVals(vals map[string]any) map[string]any {
 	return vals
 }
 
+func vendorHeader(config *Config, lang *emailLang) string { return "jfa-go" }
+func serverHeader(config *Config, lang *emailLang) string {
+	if substituteStrings == "" {
+		return "Jellyfin"
+	} else {
+		return substituteStrings
+	}
+}
+func messageFooter(config *Config, lang *emailLang) string {
+	return config.Section("messages").Key("message").String()
+}
+
 var customContent = map[string]CustomContentInfo{
 	"EmailConfirmation": {
 		Name:        "EmailConfirmation",
@@ -94,6 +106,10 @@ var customContent = map[string]CustomContentInfo{
 		Subject: func(config *Config, lang *emailLang) string {
 			return lang.InviteExpiry.get("title")
 		},
+		HeaderText: vendorHeader,
+		FooterText: func(config *Config, lang *emailLang) string {
+			return lang.InviteExpiry.get("notificationNotice")
+		},
 		Variables: []string{
 			"code",
 			"time",
@@ -131,7 +147,7 @@ var customContent = map[string]CustomContentInfo{
 			Section:       "password_resets",
 			SettingPrefix: "email_",
 			// This was the first email type added, hence the undescriptive filename.
-			DefaultValue: "email",
+			DefaultValue: "password-reset",
 		},
 	},
 	"UserCreated": {
@@ -141,6 +157,10 @@ var customContent = map[string]CustomContentInfo{
 		Subject: func(config *Config, lang *emailLang) string {
 			return lang.UserCreated.get("title")
 		},
+		HeaderText: vendorHeader,
+		FooterText: func(config *Config, lang *emailLang) string {
+			return lang.UserCreated.get("notificationNotice")
+		},
 		Variables: []string{
 			"code",
 			"name",
@@ -346,6 +366,8 @@ var EmptyCustomContent = CustomContentInfo{
 	Subject: func(config *Config, lang *emailLang) string {
 		return "EmptyCustomContent"
 	},
+	HeaderText:   serverHeader,
+	FooterText:   messageFooter,
 	Description:  nil,
 	Variables:    []string{},
 	Placeholders: map[string]any{},
@@ -359,6 +381,7 @@ var AnnouncementCustomContent = func(subject string) CustomContentInfo {
 	return cci
 }
 
+// Validates customContent and sets default fields if needed.
 var _runtimeValidation = func() bool {
 	for name, cc := range customContent {
 		if name != cc.Name {
@@ -367,6 +390,14 @@ var _runtimeValidation = func() bool {
 		if cc.DisplayName == nil {
 			panic(fmt.Errorf("no customContent[%s] DisplayName set", name))
 		}
+		if cc.HeaderText == nil {
+			cc.HeaderText = serverHeader
+			customContent[name] = cc
+		}
+		if cc.FooterText == nil {
+			cc.FooterText = messageFooter
+			customContent[name] = cc
+		}
 	}
 	return true
 }()