Rewrite cosmetics and theme preferences (#1292)

- Cosmetics and theme preferences are now only stored in cookies instead
  of a combination of both cookies and state.

- The theme plugin now supports client hints. This allows the server
  to render a page using the client-preferred theme provided it supplies
  this information (any browser other than Firefox), helping to avoid an
  annoying flash while the page is hydrating.

- The previous workaround using the Nitro plugin has been removed. Its
  functionality is now handled by the Nuxt theme plugin with cleaner
  code.

- All pages and components now use the new plugins.

apps/frontend/src/components/ui/charts/ChartDisplay.vue

@@ -162,7 +162,7 @@
                     <div
                       :style="{
                         '--color-brand': isUsingProjectColors
-                          ? intToRgba(project.color, project.id, theme ?? undefined)
+                          ? intToRgba(project.color, project.id, theme.active ?? undefined)
                           : getDefaultColor(project.id),
                       }"
                       class="legend__item__color"