Rewrite cosmetics and theme preferences (#1328)

- 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.

- For the future, the theme plugin now supports additional light themes.
  Light theme preferences are currently not stored, but this can easily
  be fixed if the need arises.

- 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.

- Compared to the previous attempt, this commit has been improved to be
  more robust in cases where the theme cookie contains invalid values.

Co-authored-by: Geometrically <18202329+Geometrically@users.noreply.github.com>

apps/frontend/src/plugins/theme/themes.ts

@@ -0,0 +1,27 @@
+export const LightThemes = ["light"] as const;
+
+export type LightTheme = (typeof LightThemes)[number];
+
+export const DarkThemes = ["dark", "oled", "retro"] as const;
+
+export type DarkTheme = (typeof DarkThemes)[number];
+
+export type Theme = LightTheme | DarkTheme;
+
+export function isLightTheme(theme: Theme | (string & Record<never, never>)): theme is LightTheme {
+  return LightThemes.includes(theme as any);
+}
+
+export function isDarkTheme(theme: Theme | (string & Record<never, never>)): theme is DarkTheme {
+  return DarkThemes.includes(theme as any);
+}
+
+export type ThemeType = "light" | "dark";
+
+export function getThemeType(
+  theme: Theme | (string & Record<never, never>),
+): ThemeType | "unknown" {
+  if (isLightTheme(theme)) return "light";
+  if (isDarkTheme(theme)) return "dark";
+  return "unknown";
+}