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" },

frontend/components/ui/badge.tsx

@@ -11,9 +11,9 @@ const badgeVariants = cva(
         secondary: "border-transparent bg-secondary text-secondary-foreground",
         destructive: "border-transparent bg-destructive text-destructive-foreground",
         outline: "text-foreground",
-        local: "border-transparent bg-blue-500/15 text-blue-500",
-        external: "border-transparent bg-emerald-500/15 text-emerald-500",
-        custom: "border-transparent bg-amber-500/15 text-amber-500",
+        local: "border-transparent bg-blue-950 text-blue-400",
+        external: "border-transparent bg-emerald-950 text-emerald-400",
+        custom: "border-transparent bg-amber-950 text-amber-400",
       },
     },
     defaultVariants: { variant: "default" },

frontend/components/ui/command.tsx

@@ -25,7 +25,7 @@ const CommandInput = React.forwardRef<
     <Search className="mr-2 h-4 w-4 shrink-0 opacity-50" />
     <CommandPrimitive.Input
       ref={ref}
-      className={cn("flex h-10 w-full rounded-md bg-transparent py-3 text-sm outline-none placeholder:text-muted-foreground disabled:cursor-not-allowed disabled:opacity-50", className)}
+      className={cn("flex h-10 w-full rounded-md bg-popover py-3 text-sm outline-none placeholder:text-muted-foreground disabled:cursor-not-allowed disabled:opacity-50", className)}
       {...props}
     />
   </div>

frontend/components/ui/dialog.tsx

@@ -17,7 +17,7 @@ const DialogOverlay = React.forwardRef<
   <DialogPrimitive.Overlay
     ref={ref}
     className={cn(
-      "fixed inset-0 z-50 bg-black/60 data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0",
+      "fixed inset-0 z-50 bg-[var(--color-overlay)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0",
       className
     )}
     {...props}

frontend/components/ui/select.tsx

@@ -16,7 +16,7 @@ const SelectTrigger = React.forwardRef<
   <SelectPrimitive.Trigger
     ref={ref}
     className={cn(
-      "flex h-9 w-full items-center justify-between gap-2 rounded-md border border-border bg-transparent px-3 py-2 text-sm shadow-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring disabled:cursor-not-allowed disabled:opacity-50 [&>span]:line-clamp-1",
+      "flex h-9 w-full items-center justify-between gap-2 rounded-md border border-border bg-background px-3 py-2 text-sm shadow-sm ring-offset-background placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-ring disabled:cursor-not-allowed disabled:opacity-50 [&>span]:line-clamp-1",
       className,
     )}
     {...props}

frontend/components/ui/sheet.tsx

@@ -16,7 +16,7 @@ const SheetOverlay = React.forwardRef<
   React.ComponentPropsWithoutRef<typeof DialogPrimitive.Overlay>
 >(({ className, ...props }, ref) => (
   <DialogPrimitive.Overlay
-    className={cn("fixed inset-0 z-50 bg-black/80 data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0", className)}
+    className={cn("fixed inset-0 z-50 bg-[var(--color-overlay)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0", className)}
     {...props}
     ref={ref}
   />