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/Sidebar.tsx

@@ -54,7 +54,6 @@ import { NavLink } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import {
   LayoutDashboard,
-  Columns3,
   FolderOpen,
   Activity,
   BarChart3,
@@ -89,10 +88,8 @@ function isUpdatePayload(x: unknown): x is UpdateStatusPayload {
 
 const NAV_KEYS = [
   { to: "/", icon: LayoutDashboard, key: "nav:dashboard" },
-  { to: "/kanban", icon: Columns3, key: "nav:agentBoard" },
   { to: "/sessions", icon: FolderOpen, key: "nav:sessions" },
   { to: "/activity", icon: Activity, key: "nav:activityFeed" },
-  { to: "/analytics", icon: BarChart3, key: "nav:analytics" },
   { to: "/workflows", icon: Workflow, key: "nav:workflows" },
   { to: "/cc-config", icon: Boxes, key: "nav:ccConfig" },
   { to: "/run", icon: Play, key: "nav:workspace" },