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/widgets/widget-form.tsx

@@ -140,7 +140,7 @@ export function WidgetForm({ widget, open, onOpenChange }: WidgetFormProps) {
                     {tz.split("/").pop()?.replace("_", " ")}
                     <button
                       type="button"
-                      className="ml-0.5 rounded-full hover:bg-foreground/10"
+                      className="ml-0.5 rounded-full hover:bg-accent"
                       onClick={() => setSelectedTzs((prev) => prev.filter((t) => t !== tz))}
                     >
                       <X className="h-2.5 w-2.5" />