refactor: centralize shared date/name helpers in utils.js (#12)

New js/utils.js exports toISO, monthPrefix, getWeekday, formatDateDE and
buildNameMap. algorithm.js, calendar.js, excel.js and history.js now import
these instead of re-implementing ISO construction, weekday math, DD.MM.YYYY
formatting and the id→name map inline. Adds tests/utils.test.mjs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -5,6 +5,6 @@
   "type": "module",
   "scripts": {
     "start": "npx serve . -p 3001",
-    "test": "node tests/algorithm.test.mjs && node tests/data.test.mjs && node tests/excel.test.mjs"
+    "test": "node tests/utils.test.mjs && node tests/algorithm.test.mjs && node tests/data.test.mjs && node tests/excel.test.mjs"
   }
 }