fix(admin): /admin.html retombait sur la page principale (ROADMAP #71)

Le menu Admin redirigeait vers /admin.html mais aucune route backend ne le
servait : le catch-all SPA /{full_path:path} renvoyait index.html, d'où le
retour à la page principale.

- backend/main.py: route GET /admin.html (gate require_admin) déclarée avant
  le catch-all SPA + correction des imports JS /frontend/js -> /static/js
- frontend/admin.html: chemins d'assets alignés sur le mount /static
- tests/test_admin.py: 3 tests de régression (page servie admin, refus 401/403)

backend/main.py

@@ -4675,6 +4675,19 @@ if FRONTEND_DIR.exists():
             return HTMLResponse(content=poc_file.read_text(encoding="utf-8"))
         raise HTTPException(status_code=404, detail="Editor POC not found")
 
+    @app.get("/admin.html", response_class=HTMLResponse)
+    async def serve_admin_page(_current_user=Depends(require_admin)):
+        """Serve the admin dashboard page (ROADMAP #71) — admin-gated.
+
+        Must be declared BEFORE the SPA catch-all ``/{full_path:path}`` or the
+        admin page would be shadowed by ``index.html`` (the reported bug: the
+        Admin menu kept returning to the main page).
+        """
+        admin_file = FRONTEND_DIR / "admin.html"
+        if admin_file.exists():
+            return HTMLResponse(content=admin_file.read_text(encoding="utf-8"))
+        raise HTTPException(status_code=404, detail="Admin page not found")
+
     @app.get("/{full_path:path}")
     async def serve_spa(full_path: str):
         """Serve the SPA index.html for all non-API routes."""