feat(demo): add demo manifest, Dutch demo entry, permanent badge and About page

Adds GET /api/v1/demo/manifest as a single source of truth for the demo's
fictional org identity (Northstar Mobility -- surfacing the project's
already-locked tenant name), synthetic-data/reset state, and live scenario
readiness. Rewrites the login screen in Dutch with an honest, no-password
demo entry and a guided-demo entry point, replaces the loud full-width
demo banner with a subtle badge + popover, and adds a compact About page
explaining what's real vs. synthetic vs. not yet connected.

frontend/e2e/_capture-screenshots.spec.ts

@@ -14,7 +14,7 @@ test("capture the seven main pages", async ({ page, request }) => {
   await page.goto("/login");
   await page.screenshot({ path: `${OUT}/1-login.png` });
 
-  await page.getByRole("button", { name: "Open as Operations Manager" }).click();
+  await page.getByRole("button", { name: "Verken als Operations Manager" }).click();
   await expect(page.getByRole("heading", { name: "Operational metrics" })).toBeVisible();
   await page.screenshot({ path: `${OUT}/2-dashboard.png`, fullPage: true });