Add Kleinunternehmerregelung (§19 UStG) support end-to-end

Checkout forces 0% VAT without de-grossing prices when the tenant is a
Kleinunternehmer (a business decision, not just an engineering default —
unlike the existing intra-community VAT exemption, which does de-gross).
Snapshotted onto the order at checkout time so a later toggle of the
company-settings checkbox never rewrites an already-issued invoice's tax
treatment — same reasoning as the existing vatExempt field.

Threaded through: checkout route, order creation/confirmation email,
on-demand invoice/Storno/Gutschrift downloads, the Bestellbestätigung
page, and the account order-detail page. The four storefront "inkl. X%
MwSt." price hints (shop grid, cart upsell, ToDo-Karten landing page,
homepage spotlight) drop that clause live when the setting is on. The
company-settings Live Preview reflects the checkbox in real time too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package-lock.json

@@ -358,8 +358,8 @@
       }
     },
     "node_modules/@einfach-produktiv/invoicing": {
-      "version": "0.1.0",
-      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#c28ceb7bd34d6297f677bf3d17aa791525654cd3",
+      "version": "0.2.1",
+      "resolved": "git+https://git.mk360.de/Marco/einfach-produktiv-invoicing.git#439165c98d6c3cc5c7c7cfe6e0a08d9242983ac8",
       "dependencies": {
         "@e-invoice-eu/core": "^3.1.1"
       },