Centralize Stylelint config and use npx

Add a dedicated duplicates Stylelint config and wire tasks to use it. Files changed:
- Added devTools/.stylelintrc.duplicates.json (no-duplicate-selectors + ignore vendored/build files).
- Updated .taskfiles/frontend.yml: added lint:css summary, run task lint:css:check with STYLELINT_CONFIG pointing at the new duplicates config, and replaced npm exec wrappers with npx stylelint.
- Extended devTools/.stylelintrc.json ignore list to include frontend/src-tauri.
- Removed obsolete frontend/stylelint.config.mjs.
This centralizes lint rules and simplifies invocation.

devTools/.stylelintrc.duplicates.json

@@ -0,0 +1,16 @@
+{
+  "ignoreFiles": [
+    "../app/core/src/main/resources/static/css/bootstrap*.css",
+    "../app/core/src/main/resources/static/css/cookieconsent.css",
+    "../app/core/src/main/resources/static/css/cookieconsentCustomisation.css",
+    "../app/core/src/main/resources/static/css/prism.css",
+    "../app/core/src/main/resources/static/pdfjs-legacy/**/*.css",
+    "../frontend/editor/dist/**/*",
+    "../frontend/editor/src-tauri/**/*",
+    "../frontend/editor/**/cookieconsent.css",
+    "../frontend/editor/**/cookieconsentCustomisation.css"
+  ],
+  "rules": {
+    "no-duplicate-selectors": true
+  }
+}

devTools/.stylelintrc.json

@@ -12,6 +12,7 @@
     "../app/core/src/main/resources/static/css/prism.css",
     "../app/core/src/main/resources/static/pdfjs-legacy/**/*.css",
     "../frontend/editor/dist/**/*",
+    "../frontend/editor/src-tauri/**/*",
     "../frontend/editor/**/cookieconsent.css",
     "../frontend/editor/**/cookieconsentCustomisation.css"
   ],