chore: cleanup dead code and duplicated logic from audit

- Remove broken webpack.yml (no package.json/webpack config exists)
- Drop dead RATE_NORMAL/RATE_WEEKEND fields on BonusCalculator; share the
  real rate constants across all three variants in variants.js
- Simplify getEmptyResult() to build allResults via .map instead of
  three manual spreads
- Replace 5 duplicated German month-name arrays and 2 weekday-name
  arrays in app.js with a shared monthName()/WEEKDAY_NAMES helper;
  drop the now-redundant getMonthName() method
- Reuse existing .modal/.modal-backdrop/.modal-content CSS classes in
  generateEmailReport() instead of hand-rolled inline styles
- Delegate ImageImporter.classify() to the shared classify() in
  variants.js instead of a duplicated copy
- Unify ad hoc YYYY-MM-DD formatting in app.js to use the existing
  HolidayProvider.formatDate() helper

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/webpack.yml

@@ -1,28 +0,0 @@
-name: NodeJS with Webpack
-
-on:
-  push:
-    branches: [ "main" ]
-  pull_request:
-    branches: [ "main" ]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-
-    strategy:
-      matrix:
-        node-version: [18.x, 20.x, 22.x]
-
-    steps:
-    - uses: actions/checkout@v4
-
-    - name: Use Node.js ${{ matrix.node-version }}
-      uses: actions/setup-node@v4
-      with:
-        node-version: ${{ matrix.node-version }}
-
-    - name: Build
-      run: |
-        npm install
-        npx webpack