feat: Split issuer address into separate fields in invoice template

Replace combined 'issuer-address' with 4 separate elements:
- issuer-street (Straße)
- issuer-house-number (Hausnummer)
- issuer-postal-code (PLZ)
- issuer-city (Ort)

Update starter layout to use new separate fields with appropriate positioning.

frontend/src/pages/InvoiceTemplatePage.tsx

@@ -49,7 +49,10 @@ const INVOICE_LOCKED_TEXT_PALETTE_IDS = new Set([
   "payment-terms",
   "bank-details",
   "issuer-name",
-  "issuer-address",
+  "issuer-street",
+  "issuer-house-number",
+  "issuer-postal-code",
+  "issuer-city",
   "issuer-contact",
 ]);
 
@@ -305,17 +308,48 @@ const INVOICE_PALETTE_ITEMS: PaletteItem[] = [
     defaultContent: (user) => user?.companyName ?? "Ihr Unternehmen",
   },
   {
-    id: "issuer-address",
+    id: "issuer-street",
     category: "issuer-data",
-    label: "Aussteller-Adresse",
-    description: "Adresse des Rechnungsausstellers",
-    width: 280,
+    label: "Aussteller-Straße",
+    description: "Straße des Rechnungsausstellers",
+    width: 200,
     fontSize: 14,
     fontWeight: 400,
     textAlign: "left",
-    defaultContent: (user) =>
-      user?.address ??
-      `${user?.street ?? ""} ${user?.houseNumber ?? ""}\n${user?.postalCode ?? ""} ${user?.city ?? ""}`,
+    defaultContent: (user) => user?.street ?? "",
+  },
+  {
+    id: "issuer-house-number",
+    category: "issuer-data",
+    label: "Aussteller-Hausnummer",
+    description: "Hausnummer des Rechnungsausstellers",
+    width: 80,
+    fontSize: 14,
+    fontWeight: 400,
+    textAlign: "left",
+    defaultContent: (user) => user?.houseNumber ?? "",
+  },
+  {
+    id: "issuer-postal-code",
+    category: "issuer-data",
+    label: "Aussteller-PLZ",
+    description: "Postleitzahl des Rechnungsausstellers",
+    width: 80,
+    fontSize: 14,
+    fontWeight: 400,
+    textAlign: "left",
+    defaultContent: (user) => user?.postalCode ?? "",
+  },
+  {
+    id: "issuer-city",
+    category: "issuer-data",
+    label: "Aussteller-Ort",
+    description: "Ort des Rechnungsausstellers",
+    width: 200,
+    fontSize: 14,
+    fontWeight: 400,
+    textAlign: "left",
+    defaultContent: (user) => user?.city ?? "",
   },
   {
     id: "issuer-contact",
@@ -950,7 +984,10 @@ function createInvoiceStarterLayout(user: UserOption | null, paletteItems: Palet
 
   return [
     createElementFromPalette(requirePaletteItem(paletteItems, "issuer-name"), user, { x: 56, y: 56 }),
-    createElementFromPalette(requirePaletteItem(paletteItems, "issuer-address"), user, { x: 56, y: 86 }),
+    createElementFromPalette(requirePaletteItem(paletteItems, "issuer-street"), user, { x: 56, y: 86 }),
+    createElementFromPalette(requirePaletteItem(paletteItems, "issuer-house-number"), user, { x: 260, y: 86 }),
+    createElementFromPalette(requirePaletteItem(paletteItems, "issuer-postal-code"), user, { x: 56, y: 110 }),
+    createElementFromPalette(requirePaletteItem(paletteItems, "issuer-city"), user, { x: 140, y: 110 }),
     createElementFromPalette(requirePaletteItem(paletteItems, "issuer-contact"), user, { x: 56, y: 130 }),
     createElementFromPalette(requirePaletteItem(paletteItems, "invoice-title"), user, { x: 480, y: 56 }),
     createElementFromPalette(requirePaletteItem(paletteItems, "invoice-number"), user, { x: 480, y: 96 }),