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/composables/theme.js

@@ -1,58 +0,0 @@
-export const useTheme = () =>
-  useState("theme", () => {
-    const colorMode = useCookie("color-mode", {
-      maxAge: 60 * 60 * 24 * 365 * 10,
-      sameSite: "lax",
-      secure: true,
-      httpOnly: false,
-      path: "/",
-    });
-
-    if (!colorMode.value) {
-      colorMode.value = {
-        value: "dark",
-        preference: "system",
-      };
-    }
-
-    if (colorMode.value.preference !== "system") {
-      colorMode.value.value = colorMode.value.preference;
-    }
-
-    return colorMode.value;
-  });
-
-export const updateTheme = (value, updatePreference = false) => {
-  const theme = useTheme();
-  const cosmetics = useCosmetics();
-
-  const themeCookie = useCookie("color-mode", {
-    maxAge: 60 * 60 * 24 * 365 * 10,
-    sameSite: "lax",
-    secure: true,
-    httpOnly: false,
-    path: "/",
-  });
-
-  if (value === "system") {
-    theme.value.preference = "system";
-
-    const colorSchemeQueryList = window.matchMedia("(prefers-color-scheme: light)");
-    if (colorSchemeQueryList.matches) {
-      theme.value.value = "light";
-    } else {
-      theme.value.value = cosmetics.value.preferredDarkTheme;
-    }
-  } else {
-    theme.value.value = value;
-    if (updatePreference) theme.value.preference = value;
-  }
-
-  if (import.meta.client) {
-    document.documentElement.className = `${theme.value.value}-mode`;
-  }
-
-  themeCookie.value = theme.value;
-};
-
-export const DARK_THEMES = ["dark", "oled", "retro"];