Add LineItem CSV import and fix Unit API

database/seeders/UnitSeeder.php

@@ -9,7 +9,7 @@ class UnitSeeder extends Seeder
 {
     public function run(): void
     {
-        Unit::create(['name' => 'Stück',    'symbol' => 'Stk']);
+        Unit::create(['name' => 'Stück',    'symbol' => 'Stk.']);
         Unit::create(['name' => 'Stunden',  'symbol' => 'h']);
         Unit::create(['name' => 'Tage',     'symbol' => 'd']);
         Unit::create(['name' => 'pauschal', 'symbol' => 'p']);