Fold the brain and health screens into Home

Home is the one overview now: the brain graph flat across the top, the flow
switches, then the health sections. The /brain and /health routes and their
sidebar entries are gone.

- charts report their cursor and clicks, so hovering one filters the list
  beside it to that minute and a click pins it until Escape or Clear
- chart values round to about three significant digits, the legend mounts
  under the plot so it can wrap without leaving the card, and axis ticks
  shorten past a thousand
- the embedded brain leaves the wheel to the page rather than zooming
- number fields no longer draw their up/down spinner (NOTEPAD)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017MeiWk3Yq12n2pTvnQWYvt

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -1,7 +1,5 @@
 import {
-  Activity,
   Bell,
-  Brain,
   Home,
   KeyRound,
   LayoutDashboard,
@@ -24,12 +22,11 @@ import useAuth from "@/hooks/useAuth"
 import { type Item, Main } from "./Main"
 
 const baseItems: Item[] = [
-  // "Home" rather than "Dashboard": dashboards are their own thing now.
+  // "Home" rather than "Dashboard": dashboards are their own thing now, and
+  // the brain and the health screens are sections of Home rather than routes.
   { icon: Home, title: "Home", path: "/" },
   { icon: Workflow, title: "Flows", path: "/flows" },
-  { icon: Brain, title: "Brain", path: "/brain" },
   { icon: LayoutDashboard, title: "Dashboards", path: "/dashboards" },
-  { icon: Activity, title: "Health", path: "/health" },
   // Both are engine-wide operator settings rather than personal ones, so they
   // sit here and not among the per-user tabs under Settings.
   { icon: KeyRound, title: "Secrets", path: "/secrets" },