Fix fonts.ts crashing every client bundle that reaches it (v0.2.9)

The v0.2.8 fileURLToPath fix (for a Turbopack asset-hash bug in the
Payload backend) wrongly assumed fonts.ts never reaches a browser
bundle. It does: einfach-produktiv's LiveCompanySettingsPreviewClient
renders <InvoiceDocument> client-side, and several Client Components
pull this module in transitively via the index.ts barrel just by
importing computeTaxBreakdown. fileURLToPath isn't a real function in
a browser's node:url shim, so calling it unconditionally at module
scope crashed module evaluation for any client bundle that included
it — including einfach-produktiv's entire /cart page.

Fix: branch on typeof window === "undefined" — browser keeps the
original new URL(..., import.meta.url) idiom (correctly asset-hashed
by Turbopack for a consumer's own build), Node.js keeps the
fileURLToPath/path.join resolution from v0.2.8.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.8",
+  "version": "0.2.9",
   "private": true,
   "description": "Shared invoice / correction-invoice (Stornorechnung, Gutschrift) PDF generation and VAT-breakdown math, consumed as a git dependency by both the einfach-produktiv frontend and the payload backend — not published to npm.",
   "type": "module",