Merge feature/bonus-varianten: 3 variants + vacation + date-stepper

Conflicts resolved:
- sw.js: bumped CACHE_NAME to dienstplan-pro-v4 (was v3 + v2). Both
  variants.js and image-import.js are in ASSETS.
- storage.js: kept STORAGE_KEY_DUTIES + STORAGE_KEY_VACATION (Feature B)
  alongside STORAGE_KEY_OPENROUTER_KEY/MODEL + DEFAULT_MODEL (Feature A).
- styles.css: appended Feature B variants/vacation/date-stepper rules
  after Feature A modal/key rules; both blocks coexist.

test.html

@@ -140,6 +140,7 @@
     <div id="test-results"></div>
 
     <script src="holidays.js"></script>
+    <script src="variants.js"></script>
     <script src="calculator.js"></script>
     <script src="storage.js"></script>
     <script src="image-import.js"></script>