refactor(frontend): simplify theme system and unify UI components

Remove the "casaos" theme in favor of a unified design system. This involves cleaning up conditional styling across components, simplifying the theme toggle, and updating the global CSS variables to a more consistent dark/light mode implementation.

- Remove `casaos` theme from `themes.ts` and `ThemeToggle`
- Refactor `globals.css` to use a single dark mode definition
- Simplify component styling by removing `isCasaOS` conditional logic
- Update UI components (`Card`, `Badge`, `WidgetCard`, etc.) to use standard design tokens
- Update E2E smoke tests to reflect theme changes

frontend/lib/theme/themes.ts

@@ -1,11 +1,11 @@
-export type Theme = "light" | "dark" | "casaos";
+export type Theme = "light" | "dark";
 
 const STORAGE_KEY = "dash-theme";
 
 export function getStoredTheme(): Theme {
   if (typeof window === "undefined") return "dark";
   const stored = localStorage.getItem(STORAGE_KEY);
-  if (stored === "light" || stored === "dark" || stored === "casaos") return stored;
+  if (stored === "light" || stored === "dark") return stored;
   return "dark";
 }
 
@@ -22,5 +22,4 @@ export function applyTheme(theme: Theme) {
 export const themeLabels: Record<Theme, string> = {
   light: "Light",
   dark: "Dark",
-  casaos: "CasaOS",
 };