feat(dashboard): merge Analytics into Dashboard, remove Kanban Board

Closes issue #5 (Dashboard | Update menu & page):
- Analytics content (heatmap, sparkline, token/cost/workflow/productivity
  charts) moves into a new "Analytics" tab on the Dashboard page, alongside
  the existing Monitor/Health tabs. The standalone Analytics page, its
  /analytics route, and its sidebar menu entry are removed.
- The Kanban Board page, its /kanban route, and its sidebar menu entry are
  removed entirely (page content deleted, not just hidden).
- Updated ARCHITECTURE.md, README.md, client/README.md, docs/API.md, and
  the affected client tests/snapshots to match.

client/src/components/__tests__/Sidebar.test.tsx

@@ -32,7 +32,6 @@ describe("Sidebar", () => {
   it("should render all navigation links", () => {
     renderSidebar(true);
     expect(screen.getByText("Dashboard")).toBeInTheDocument();
-    expect(screen.getByText("Kanban Board")).toBeInTheDocument();
     expect(screen.getByText("Sessions")).toBeInTheDocument();
     expect(screen.getByText("Activity Feed")).toBeInTheDocument();
   });
@@ -60,7 +59,6 @@ describe("Sidebar", () => {
     const links = screen.getAllByRole("link");
     const hrefs = links.map((link) => link.getAttribute("href"));
     expect(hrefs).toContain("/");
-    expect(hrefs).toContain("/kanban");
     expect(hrefs).toContain("/sessions");
     expect(hrefs).toContain("/activity");
   });
@@ -82,7 +80,6 @@ describe("Sidebar", () => {
 
     await waitFor(() => {
       expect(screen.getByText("Tổng quan")).toBeInTheDocument();
-      expect(screen.getByText("Bảng Kanban")).toBeInTheDocument();
     });
   });