fix(ui): final cleanup — signal chart hex, checkbox/select tokens

- SignalHistoryChart: #3b82f6 → hsl(var(--accent))
- Checkbox: opacity-50 disabled → text-muted/border-subtle, proper
  focus ring, bg-panel surface, checked text-background
- Select: opacity-50 disabled → text-muted/border-subtle, proper
  focus ring, radius-control, bg-panel, text-xs

Zero old token names, zero blue hex, zero opacity-disabled remaining.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/ui/checkbox.tsx

@@ -10,7 +10,7 @@ const Checkbox = React.forwardRef<
   <CheckboxPrimitive.Root
     ref={ref}
     className={cn(
-      'peer h-4 w-4 shrink-0 rounded-sm border border-border bg-elevated/50 focus-visible:outline-none focus-visible:border-accent disabled:cursor-not-allowed disabled:opacity-50 data-[state=checked]:bg-accent data-[state=checked]:border-accent data-[state=checked]:text-white',
+      'peer h-4 w-4 shrink-0 rounded-sm border border-border-default bg-panel focus-visible:outline-2 focus-visible:outline-accent focus-visible:outline-offset-2 disabled:cursor-not-allowed disabled:text-text-muted disabled:border-border-subtle data-[state=checked]:bg-accent data-[state=checked]:border-accent data-[state=checked]:text-background',
       className,
     )}
     {...props}

frontend/src/components/ui/select.tsx

@@ -14,7 +14,7 @@ const SelectTrigger = React.forwardRef<
   <SelectPrimitive.Trigger
     ref={ref}
     className={cn(
-      'flex h-8 w-full items-center justify-between rounded-md border border-border bg-elevated/50 px-3 py-1 text-sm text-text-primary focus:outline-none focus:border-accent disabled:cursor-not-allowed disabled:opacity-50 [&>span]:line-clamp-1',
+      'flex h-8 w-full items-center justify-between rounded-[var(--radius-control)] border border-border-default bg-panel px-2.5 py-1 text-xs text-text-primary focus-visible:outline-2 focus-visible:outline-accent focus-visible:outline-offset-2 disabled:cursor-not-allowed disabled:text-text-muted disabled:border-border-subtle [&>span]:line-clamp-1',
       className,
     )}
     {...props}

frontend/src/components/wireless/SignalHistoryChart.tsx

@@ -129,7 +129,7 @@ export function SignalHistoryChart({ tenantId, deviceId, macAddress }: SignalHis
             <Line
               type="monotone"
               dataKey="signal_avg"
-              stroke="#3b82f6"
+              stroke="hsl(var(--accent))"
               strokeWidth={2}
               dot={false}
               activeDot={{ r: 3 }}