fix(ui): light mode visibility + sidebar layout

- Replace accent with primary for upgrade banner (invisible on white bg)
- Fix sidebar actions (theme/logout) stuck at bottom with mt-auto
- Use primary color for user avatar fallback in light mode

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/dashboard/profile/page.tsx

@@ -421,10 +421,10 @@ export default function ProfilePage() {
                 )}
 
                 {isFreePlan && (
-                  <div className="flex items-center gap-3 p-3 rounded-lg bg-accent/10 border border-accent/20 text-sm">
-                    <Zap className="w-4 h-4 text-accent flex-shrink-0" />
-                    <span className="text-accent/90 flex-1">Débloquez plus de traductions avec un forfait payant.</span>
-                    <Button asChild size="sm" variant="outline" className="border-accent/30 text-accent hover:bg-accent/10 flex-shrink-0">
+                  <div className="flex items-center gap-3 p-3 rounded-lg bg-primary/10 border border-primary/20 text-sm">
+                    <Zap className="w-4 h-4 text-primary flex-shrink-0" />
+                    <span className="text-primary flex-1">Débloquez plus de traductions avec un forfait payant.</span>
+                    <Button asChild size="sm" variant="outline" className="border-primary/30 text-primary hover:bg-primary/10 flex-shrink-0">
                       <Link href="/pricing">Voir les forfaits <ArrowRight className="w-3.5 h-3.5 ml-1" /></Link>
                     </Button>
                   </div>