Fix font path breaking every PDF render inside the Payload backend (v0.2.8)

fonts.ts resolved the vendored Liberation Sans .ttf files via
`new URL('./assets/fonts/...', import.meta.url).pathname` — Next.js's
Turbopack/webpack bundler statically rewrites exactly that expression
into its own hashed static-asset system. Harmless in the frontend's own
build, but the Payload backend is also a Next.js app consuming this
package as a node_modules git dependency, where the hash referenced in
the compiled server code didn't match what was actually emitted to disk
— every invoice/correction-invoice admin download failed with
ENOENT: .../LiberationSans-Regular.<hash>.ttf. Not a cache issue,
reproduced identically after a from-scratch --no-cache rebuild. Fixed by
resolving the path via fileURLToPath(import.meta.url) + path.join
instead, which Turbopack does not special-case.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@einfach-produktiv/invoicing",
-  "version": "0.2.7",
+  "version": "0.2.8",
   "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",