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/pages/__tests__/screens.snapshot.test.tsx

@@ -429,11 +429,9 @@ vi.mock("../../lib/push", () => ({
 
 // Page components (imported after the mocks above; vi.mock is hoisted).
 import { Dashboard } from "../Dashboard";
-import { KanbanBoard } from "../KanbanBoard";
 import { Sessions } from "../Sessions";
 import { SessionDetail } from "../SessionDetail";
 import { ActivityFeed } from "../ActivityFeed";
-import { Analytics } from "../Analytics";
 import { Workflows } from "../Workflows";
 import { CcConfig } from "../CcConfig";
 import { Workspace } from "../Workspace";
@@ -511,9 +509,6 @@ describe("screen snapshots", () => {
   it("Dashboard", async () => {
     await snapshot(<Dashboard />, "/");
   });
-  it("Kanban board", async () => {
-    await snapshot(<KanbanBoard />, "/kanban");
-  });
   it("Sessions", async () => {
     await snapshot(<Sessions />, "/sessions");
   });
@@ -528,9 +523,6 @@ describe("screen snapshots", () => {
   it("Activity feed", async () => {
     await snapshot(<ActivityFeed />, "/activity");
   });
-  it("Analytics", async () => {
-    await snapshot(<Analytics />, "/analytics");
-  });
   it("Workflows", async () => {
     await snapshot(<Workflows />, "/workflows");
   });