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/index.css

@@ -173,6 +173,18 @@
   [role="button"] {
     cursor: pointer;
   }
+  /* Number fields are typed into, not clicked up and down one step at a time,
+     and the spinner crowds the pill. */
+  input[type="number"] {
+    -moz-appearance: textfield;
+    appearance: textfield;
+  }
+  input[type="number"]::-webkit-outer-spin-button,
+  input[type="number"]::-webkit-inner-spin-button {
+    -webkit-appearance: none;
+    appearance: none;
+    margin: 0;
+  }
 }
 
 /* Display face for headings / wordmarks. Plain class so it stays outside the