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/components/ui/alert.tsx

@@ -8,7 +8,7 @@ const alertVariants = cva(
     variants: {
       variant: {
         default: "bg-background text-foreground",
-        destructive: "border-destructive/50 text-destructive dark:border-destructive [&>svg]:text-destructive",
+        destructive: "border-destructive text-destructive [&>svg]:text-destructive",
       },
     },
     defaultVariants: { variant: "default" },