fix(ui): Lot A #151 #158 #159 (QA verte)

- #151: chevauchement des boutons dans la barre supérieure pendant une conversation agent

- #158: popup de reprise de conversation intempestive (ajout cellule / changement layout / projet)

- #159: pastille d'activité de projet

frontend/src/shared/ui/Tabs.tsx

@@ -7,12 +7,15 @@
 
 import { cn } from "../lib/cn";
 import { IconButton } from "./IconButton";
+import type { ReactNode } from "react";
 
 export interface TabItem {
   /** Stable id, returned by `onSelect`/`onClose`. */
   id: string;
   /** Visible label. */
   label: string;
+  /** Optional leading visual, such as a status indicator. */
+  leading?: ReactNode;
 }
 
 export interface TabsProps {
@@ -52,11 +55,12 @@ export function Tabs({ items, value, onSelect, onClose, className }: TabsProps)
               aria-selected={active}
               onClick={() => onSelect(tab.id)}
               className={cn(
-                "px-2 py-1 text-sm focus-visible:outline-none",
+                "flex items-center gap-1.5 px-2 py-1 text-sm focus-visible:outline-none",
                 active ? "font-semibold text-content" : "text-muted hover:text-content",
               )}
             >
-              {tab.label}
+              {tab.leading}
+              <span>{tab.label}</span>
             </button>
             {onClose && (
               <IconButton