Rename project from numbers-study to numolus

- package.json / package-lock.json: package name "numolus"
- README: new title, drop the "Case- und Designstudie" framing, update
  test count (331 -> 528)
- localStorage keys renamed to numolus:doc:v2 / numolus:theme; the old
  numbers-study:* keys are still read as a fallback so existing documents
  and theme choices survive the rename

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

package-lock.json

@@ -1,11 +1,11 @@
 {
-  "name": "numbers-study",
+  "name": "numolus",
   "version": "1.0.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "numbers-study",
+      "name": "numolus",
       "version": "1.0.0",
       "dependencies": {
         "chart.js": "^4.4.6",