feat(ui): refonte menus & fenêtres — menu unique Panneaux et onglets projet (#26)

Sous-menus flyout dans MenuBar ; ProjectsView expose un menu unique
Panneaux et supprime les menus View/Window/File ainsi que le sélecteur
de projet ; ProjectTabs gère les onglets et le bouton +. Tests migrés
en conséquence.

QA vert : tsc --noEmit exit 0, vitest 62 fichiers / 616 tests passés,
invariants préservés.

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

frontend/src/features/projects/ProjectTabs.tsx

@@ -1,9 +1,13 @@
 /**
  * `ProjectTabs` — the project tab bar rendered below the app header.
  *
- * Shows one tab per open project with a close control, and a "+" button to
- * return to the launcher.  Purely presentational: all state lives in the
- * parent (ProjectsView via useProjects).
+ * Shows one tab per open project with a close control, and a "+" button to the
+ * right that opens the **Projects** panel (floating) — the single entry point
+ * for creating a project or switching to a known one (#26). Purely
+ * presentational: all state lives in the parent (ProjectsView via useProjects).
+ *
+ * The bar is always present (even with no open project) so the "+" stays
+ * reachable; the empty case shows a muted placeholder next to it.
  */
 
 import type { TabItem } from "@/shared";
@@ -14,9 +18,10 @@ export interface ProjectTabsProps {
   activeTabId: string | null;
   onSelect: (id: string) => void;
   onClose: (id: string) => void;
-  /** Whether the launcher overlay is currently visible (no active tab). */
-  showingLauncher: boolean;
-  onShowLauncher: () => void;
+  /** Whether the Projects panel is currently open (in any placement). */
+  projectsPanelOpen: boolean;
+  /** Open the Projects panel (floating) — create/switch project. */
+  onOpenProjectsPanel: () => void;
   className?: string;
 }
 
@@ -25,31 +30,33 @@ export function ProjectTabs({
   activeTabId,
   onSelect,
   onClose,
-  showingLauncher,
-  onShowLauncher,
+  projectsPanelOpen,
+  onOpenProjectsPanel,
   className,
 }: ProjectTabsProps) {
-  if (items.length === 0) return null;
-
   return (
     <div
       className={cn(
-        "flex items-center gap-1 border-b border-border bg-surface px-2 py-1",
+        "flex shrink-0 items-center gap-1 border-b border-border bg-surface px-2 py-1",
         className,
       )}
     >
-      <Tabs
-        items={items}
-        value={activeTabId}
-        onSelect={onSelect}
-        onClose={onClose}
-        className="flex-1"
-      />
+      {items.length === 0 ? (
+        <p className="flex-1 px-2 text-sm text-muted">No open tabs.</p>
+      ) : (
+        <Tabs
+          items={items}
+          value={activeTabId}
+          onSelect={onSelect}
+          onClose={onClose}
+          className="flex-1"
+        />
+      )}
       <Button
         size="sm"
-        variant={showingLauncher ? "secondary" : "ghost"}
-        onClick={onShowLauncher}
-        aria-label="open project launcher"
+        variant={projectsPanelOpen ? "secondary" : "ghost"}
+        onClick={onOpenProjectsPanel}
+        aria-label="open projects panel"
         className="shrink-0"
       >
         +