feat(email): default template with inline logo + shop contact vars

Mirrors the layout from data/mail_template.png:
- Company name + greeting headline
- Body referencing the invoice number
- Inline logo (cid:invoice-logo) attached automatically
- Footer with mailto + website links

New template vars: {{shopEmail}}, {{shopWebsite}}.
Settings UI prefills empty fields with the defaults so users see and
can tweak them without losing the fallback.

app/routes/app.settings.tsx

@@ -14,6 +14,12 @@ import {
   storeUploadedLogo,
 } from "../services/invoice/logoCache.server";
 import { RichTextEditor } from "../components/RichTextEditor";
+import {
+  DEFAULT_EMAIL_BODY_DE,
+  DEFAULT_EMAIL_BODY_EN,
+  DEFAULT_EMAIL_SUBJECT_DE,
+  DEFAULT_EMAIL_SUBJECT_EN,
+} from "../services/invoice/emailTemplates";
 
 interface SettingsFieldErrors {
   vatId?: string;
@@ -376,13 +382,13 @@ export default function SettingsRoute() {
             <Field
               label="Subject (German)"
               name="emailSubjectDe"
-              defaultValue={settings.emailSubjectDe}
+              defaultValue={settings.emailSubjectDe || DEFAULT_EMAIL_SUBJECT_DE}
               helpText="Variables like {{invoiceNumber}} are substituted at send time."
             />
             <RichTextEditor
               name="emailBodyHtmlDe"
               label="Body (German)"
-              defaultValue={settings.emailBodyHtmlDe}
+              defaultValue={settings.emailBodyHtmlDe || DEFAULT_EMAIL_BODY_DE}
               variables={EMAIL_VARS}
               minHeight={220}
             />
@@ -390,13 +396,13 @@ export default function SettingsRoute() {
             <Field
               label="Subject (English)"
               name="emailSubjectEn"
-              defaultValue={settings.emailSubjectEn}
+              defaultValue={settings.emailSubjectEn || DEFAULT_EMAIL_SUBJECT_EN}
               helpText="Variables like {{invoiceNumber}} are substituted at send time."
             />
             <RichTextEditor
               name="emailBodyHtmlEn"
               label="Body (English)"
-              defaultValue={settings.emailBodyHtmlEn}
+              defaultValue={settings.emailBodyHtmlEn || DEFAULT_EMAIL_BODY_EN}
               variables={EMAIL_VARS}
               minHeight={220}
             />
@@ -425,6 +431,8 @@ const EMAIL_VARS = [
   { token: "{{dueDate}}" },
   { token: "{{companyName}}" },
   { token: "{{ownerName}}" },
+  { token: "{{shopEmail}}" },
+  { token: "{{shopWebsite}}" },
 ];
 
 interface FieldProps {