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>

variants.js

@@ -4,7 +4,9 @@
  * Loaded after holidays.js and before calculator.js.
  */
 
-// Will be implemented in subsequent tasks.
+const RATE_NORMAL  = 250;
+const RATE_WEEKEND = 450;
+
 function classify(date, holidayProvider) {
     const wd = date.getDay(); // 0=So, 1=Mo, ..., 5=Fr, 6=Sa
 
@@ -34,8 +36,6 @@ function classifyDuties(duties, holidayProvider) {
 }
 
 function variant1(classified, isVacation) {
-    const RATE_NORMAL  = 250;
-    const RATE_WEEKEND = 450;
     const frSoThreshold    = isVacation ? 0.5 : 1;
     const weekdayThreshold = isVacation ? 1.5 : 3;
     const frSoDeduction    = isVacation ? 0.5 : 1;
@@ -89,8 +89,6 @@ function variant1(classified, isVacation) {
 }
 
 function variant2(classified, isVacation) {
-    const RATE_NORMAL  = 250;
-    const RATE_WEEKEND = 450;
     const saThreshold      = isVacation ? 0.5 : 1;
     const weekdayThreshold = isVacation ? 1   : 2;
     const saDeduction      = isVacation ? 0.5 : 1;
@@ -135,8 +133,6 @@ function variant2(classified, isVacation) {
 }
 
 function variant3(classified, isVacation) {
-    const RATE_NORMAL  = 250;
-    const RATE_WEEKEND = 450;
     const poolThreshold = isVacation ? 1 : 2;
     const totalDeduction = isVacation ? 1 : 2;