feat(web): add Seedance 2.0 promo banner and modal flow (#749)

* feat(web): add Seedance 2.0 promo banner and modal flow

Add a dismiss-once promo banner on Home (both idle and active states)
and a two-step modal: GitHub Star → join Feishu group to apply for a
Seedance 2.0 experience Key. Visual style aligned with the prototype
in refly-ai/agent-digital-cowork PR #119 and #120.

* fix(web): update Seedance tutorial URL to docs.nexu.io

* fix(web): resolve biome a11y lint errors in seedance promo

Author: lefarcen

apps/web/src/components/seedance-promo.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+import { getSeedancePromoCountdown } from "./seedance-promo";
+
+describe("getSeedancePromoCountdown", () => {
+  it("starts each cycle at two days remaining", () => {
+    const cycleStart = new Date("2026-04-01T00:00:00+08:00");
+    const now = cycleStart.getTime();
+
+    expect(getSeedancePromoCountdown(now, cycleStart)).toMatchObject({
+      days: 1,
+      hours: 23,
+      minutes: 59,
+    });
+  });
+
+  it("loops back after two days", () => {
+    const cycleStart = new Date("2026-04-01T00:00:00+08:00");
+    const now = new Date("2026-04-03T01:30:00+08:00").getTime();
+
+    expect(getSeedancePromoCountdown(now, cycleStart)).toMatchObject({
+      days: 1,
+      hours: 22,
+      minutes: 30,
+    });
+  });
+});