fix(projects): rendre visible le bouton + d'ajout d'onglet projet (#46)

Le bouton + de la barre d'onglets projets était invisible (pas de variant
d'affichage). Il est désormais rendu en permanence via un variant secondary,
avec aria-pressed et title pour l'accessibilité, permettant d'ajouter un
projet en onglet.

Couvert par un nouveau test (ProjectTabs.test.tsx).

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

frontend/src/features/projects/ProjectTabs.test.tsx

@@ -0,0 +1,76 @@
+/**
+ * #46 — the project tab bar's "+" add-project button. It must be present,
+ * enabled and visibly a button in EVERY state (notably with zero open projects,
+ * where it is the only way to add a project as a tab), and clicking it opens the
+ * Projects panel. The visibility regression it guards against: the button used
+ * to render with the `ghost` variant (no border/background, muted text) so it
+ * read as faint text rather than a control.
+ */
+import { describe, it, expect, vi } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+
+import type { TabItem } from "@/shared";
+import { ProjectTabs } from "./ProjectTabs";
+
+const TABS: TabItem[] = [
+  { id: "a", label: "alpha" },
+  { id: "b", label: "beta" },
+];
+
+function renderTabs(props: Partial<React.ComponentProps<typeof ProjectTabs>> = {}) {
+  const onOpenProjectsPanel = vi.fn();
+  render(
+    <ProjectTabs
+      items={props.items ?? TABS}
+      activeTabId={props.activeTabId ?? "a"}
+      onSelect={props.onSelect ?? vi.fn()}
+      onClose={props.onClose ?? vi.fn()}
+      projectsPanelOpen={props.projectsPanelOpen ?? false}
+      onOpenProjectsPanel={onOpenProjectsPanel}
+    />,
+  );
+  return { onOpenProjectsPanel };
+}
+
+describe("ProjectTabs add-project button (#46)", () => {
+  it("renders the + button when projects are open", () => {
+    renderTabs({ items: TABS });
+    const btn = screen.getByRole("button", { name: "open projects panel" });
+    expect(btn).toBeTruthy();
+    expect(btn.textContent).toContain("+");
+    expect((btn as HTMLButtonElement).disabled).toBe(false);
+  });
+
+  it("still renders the + button with ZERO open projects (only way to add one)", () => {
+    renderTabs({ items: [], activeTabId: null });
+    // The empty placeholder is shown…
+    expect(screen.getByText("No open tabs.")).toBeTruthy();
+    // …and the add-project button is still present and enabled.
+    const btn = screen.getByRole("button", { name: "open projects panel" });
+    expect(btn).toBeTruthy();
+    expect((btn as HTMLButtonElement).disabled).toBe(false);
+  });
+
+  it("is a visible bordered control, not the near-invisible ghost glyph", () => {
+    // Regression guard for #46: the secondary variant carries a border; the old
+    // ghost variant did not. Asserting the border class keeps the button from
+    // silently reverting to the invisible style.
+    renderTabs({ items: [] });
+    const btn = screen.getByRole("button", { name: "open projects panel" });
+    expect(btn.className).toContain("border");
+    expect(btn.className).not.toContain("text-muted");
+  });
+
+  it("reflects the panel-open state via aria-pressed", () => {
+    const { onOpenProjectsPanel } = renderTabs({ projectsPanelOpen: true });
+    const btn = screen.getByRole("button", { name: "open projects panel" });
+    expect(btn.getAttribute("aria-pressed")).toBe("true");
+    expect(onOpenProjectsPanel).not.toHaveBeenCalled();
+  });
+
+  it("opens the Projects panel on click, independent of any active project", () => {
+    const { onOpenProjectsPanel } = renderTabs({ items: [], activeTabId: null });
+    fireEvent.click(screen.getByRole("button", { name: "open projects panel" }));
+    expect(onOpenProjectsPanel).toHaveBeenCalledTimes(1);
+  });
+});

frontend/src/features/projects/ProjectTabs.tsx

@@ -52,12 +52,21 @@ export function ProjectTabs({
           className="flex-1"
         />
       )}
+      {/* Add-project affordance (#46): always a bordered `secondary` button so
+          it reads as a clickable control in every state — including zero open
+          projects — instead of the near-invisible `ghost` glyph it used to be.
+          When the Projects panel is open it also shows the pressed ring. */}
       <Button
         size="sm"
-        variant={projectsPanelOpen ? "secondary" : "ghost"}
+        variant="secondary"
         onClick={onOpenProjectsPanel}
         aria-label="open projects panel"
-        className="shrink-0"
+        aria-pressed={projectsPanelOpen}
+        title="Ajouter un projet"
+        className={cn(
+          "shrink-0",
+          projectsPanelOpen && "ring-1 ring-border-strong",
+        )}
       >
         +
       </Button>