feat: Phase 2+3 frontend coverage — responsive + accessibility tests + CI threshold

- Viewer accessibility: 5 tests (aria-labels, aria-expanded, label association, alert role)
- Viewer responsive: 3 tests (chat-collapsed, CSS grid, now-playing aria-live)
- Admin accessibility: 3 tests (nav aria-label, aria-current, focusability)
- Admin responsive: 2 tests (tab visibility, essential tabs)
- Coverage: vitest v8 provider, 40% stmt/lines, 30% branches/funcs
- CI: test:coverage step between lint and build
- Total: 44 frontend tests (31 + 13 new)
- Updated TODO.md (Phase 2+3 done), DONE.md, MEMORY.md

frontend/src/admin.test.ts

@@ -271,3 +271,58 @@ file '/app/media/demo2.mp4'
     expect(list.innerHTML).toContain("demo1.mp4");
   });
 });
+
+describe("admin accessibility", () => {
+  beforeEach(() => {
+    document.body.innerHTML = `
+      <nav class="admin-nav" aria-label="Admin navigation">
+        <button data-tab="dashboard" aria-current="page">Dashboard</button>
+        <button data-tab="playlist">Playlist</button>
+        <button data-tab="users">Users</button>
+        <button data-tab="moderation">Moderation</button>
+      </nav>
+      <div id="tab-content"></div>
+    `;
+  });
+
+  it("nav has aria-label for screen readers", () => {
+    const nav = document.querySelector(".admin-nav");
+    expect(nav?.getAttribute("aria-label")).toBe("Admin navigation");
+  });
+
+  it("active tab has aria-current attribute", () => {
+    const active = document.querySelector("[aria-current]");
+    expect(active).toBeTruthy();
+    expect(active?.getAttribute("aria-current")).toBe("page");
+  });
+
+  it("all nav buttons are focusable", () => {
+    const buttons = document.querySelectorAll(".admin-nav button");
+    buttons.forEach((btn) => {
+      expect(btn.getAttribute("tabindex")).toBeNull();
+      expect(btn.getAttribute("disabled")).toBeNull();
+    });
+  });
+});
+
+describe("admin responsive layout", () => {
+  beforeEach(() => {
+    document.body.innerHTML = `
+      <nav class="admin-nav"><button data-tab="dashboard">Dashboard</button></nav>
+      <div id="tab-content"><div id="overview-content">Overview data</div></div>
+    `;
+  });
+
+  it("tab content is present and visible", () => {
+    const tabContent = document.getElementById("tab-content");
+    expect(tabContent).toBeTruthy();
+    const style = window.getComputedStyle(tabContent!);
+    expect(style.display).not.toBe("none");
+  });
+
+  it("admin nav contains essential tabs", () => {
+    const buttons = document.querySelectorAll(".admin-nav button");
+    const tabs = Array.from(buttons).map((b) => b.textContent?.trim());
+    expect(tabs).toContain("Dashboard");
+  });
+});