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/Flow/flow.css

@@ -112,6 +112,14 @@
   transform: translate(-50%, -50%);
 }
 
+/*
+ * Embedded on Home rather than on a canvas of its own: the graph paints no
+ * surface, so the page's own background shows through behind the neurons.
+ */
+.react-flow.brain-flat {
+  --xy-background-color: transparent;
+}
+
 .brain-cell {
   cursor: pointer;
   transition: border-color var(--duration-base) var(--ease-standard);