Merge feature/ticket52-add-project-button-tabs into develop

Ticket #52 : le bouton « + » d'ajout de projet est collé aux onglets
(retrait des `flex-1`). Validé QA vert (typecheck + build + 665 tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/features/projects/ProjectTabs.tsx

@@ -42,14 +42,13 @@ export function ProjectTabs({
       )}
     >
       {items.length === 0 ? (
-        <p className="flex-1 px-2 text-sm text-muted">No open tabs.</p>
+        <p className="px-2 text-sm text-muted">No open tabs.</p>
       ) : (
         <Tabs
           items={items}
           value={activeTabId}
           onSelect={onSelect}
           onClose={onClose}
-          className="flex-1"
         />
       )}
       {/* Add-project affordance (#46): always a bordered `secondary` button so