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.

sw.js

@@ -1,10 +1,11 @@
-const CACHE_NAME = 'dienstplan-pro-v3';
+const CACHE_NAME = 'dienstplan-pro-v4';
 const ASSETS = [
   './',
   './index.html',
   './styles.css',
   './app.js',
   './calculator.js',
+  './variants.js',
   './holidays.js',
   './storage.js',
   './image-import.js'