feat(email): WYSIWYG template editor with variable substitution

- Add emailSubject{De,En} + emailBodyHtml{De,En} to ShopSettings
- New RichTextEditor component (TipTap) with toolbar + variable insert
- Settings UI: Email templates section per language
- email.server.ts: substitute {{var}} placeholders, fall back to defaults
- Default vars: invoiceNumber, customerName, customerFirstName, orderName,
  totalGross, dueDate, companyName, ownerName

app/routes/app.invoices.tsx

@@ -165,8 +165,8 @@ export default function InvoicesPage() {
         </s-stack>
 
         {isLoading ? (
-          <s-stack direction="inline" gap="small" alignItems="center">
-            <s-spinner size="small" accessibilityLabel="Loading orders" />
+          <s-stack direction="inline" gap="base" alignItems="center">
+            <s-spinner size="base" accessibilityLabel="Loading orders" />
             <s-text tone="neutral">Loading…</s-text>
           </s-stack>
         ) : orders.length === 0 ? (