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/lib/utils.ts

@@ -4,3 +4,13 @@ import { twMerge } from "tailwind-merge"
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs))
 }
+
+/**
+ * A reading at about three significant digits: 203.63 → 204, 2.714 → 2.7.
+ *
+ * Shared by the health tables and the chart legends, so a value read off the
+ * cursor cannot grow wide enough to push a legend out of its card.
+ */
+export function compact(value: number): string {
+  return String(Math.abs(value) >= 100 ? Math.round(value) : +value.toFixed(1))
+}