build(deps): bump zod from 3.25.76 to 4.4.3 in /backend

* build(deps): bump zod from 3.25.76 to 4.4.3 in /backend

Bumps [zod](https://github.com/colinhacks/zod) from 3.25.76 to 4.4.3.
- [Release notes](https://github.com/colinhacks/zod/releases)
- [Commits](https://github.com/colinhacks/zod/compare/v3.25.76...v4.4.3)

---
updated-dependencies:
- dependency-name: zod
  dependency-version: 4.4.3
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

* fix: adapt backend validation for zod v4

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Daniel Volz <mail@danielvolz.org>

backend/src/routes/share.ts

@@ -385,7 +385,7 @@ export async function shareRoutes(app: FastifyInstance) {
 			const parsed = createShareSchema.safeParse(request.body);
 			if (!parsed.success) {
 				return reply.status(400).send({
-					error: parsed.error.errors[0]?.message ?? "Invalid input",
+					error: parsed.error.issues[0]?.message ?? "Invalid input",
 					code: "VALIDATION_ERROR",
 				});
 			}