feat: replace histoire with a minimal in-house stories harness

Histoire is unmaintained and blocks upgrading Vite and other dev
dependencies. Replace it with a small standalone Vite app that keeps the
same `<Story>`/`<Variant>` template API so story files need no changes.

- Add vite.stories.config.ts and app/javascript/stories/ (sidebar tree
  with collapsible groups, search, dark-mode and RTL/LTR toggles, and a
  per-story error boundary)
- Globally register Story/Variant; migrate the histoire setup (store,
  i18n, plugins/directives) and seed a memory router for route-aware
  components
- Repoint story:dev/build/preview scripts and drop the histoire deps
- Remove histoire.config.ts, histoire.setup.ts and histoire.scss

app/javascript/design-system/histoire.scss

@@ -1,21 +0,0 @@
-@import 'dashboard/assets/scss/app';
-
-*,
-::before,
-::after {
-  --_histoire-color-primary-50: 235 245 255;
-  --_histoire-color-primary-100: 194 225 255;
-  --_histoire-color-primary-200: 153 206 255;
-  --_histoire-color-primary-300: 112 186 255;
-  --_histoire-color-primary-400: 71 166 255;
-  --_histoire-color-primary-500: 31 147 255;
-  --_histoire-color-primary-600: 25 118 204;
-  --_histoire-color-primary-700: 19 88 153;
-  --_histoire-color-primary-800: 12 59 102;
-  --_histoire-color-primary-900: 6 29 51;
-}
-
-html,
-body {
-  font-family: 'Inter', sans-serif;
-}

app/javascript/design-system/stories.scss

@@ -0,0 +1,6 @@
+@import 'dashboard/assets/scss/app';
+
+html,
+body {
+  font-family: 'Inter', sans-serif;
+}