Draw every flow as one graph, merged on what it talks to

A node type can now say which outside thing its parameters point at, and
nodes sharing one — a broker topic, a URL, a bucket — are drawn as a single
neuron on a new /brain canvas. That makes the wiring which runs between
flows through a broker visible for the first time; no single flow's canvas
can show it. The key is read off stored parameters, so a credential
reference never reaches an id.

Layout is a d3 force simulation settled once and then frozen, lit by the
socket the editor already listens to: a neuron pulses when any node behind
it publishes, and its connections light as values pass.

Fixes the message pulse while here: interpolating the stroke against the
edge's `color-mix()` resting colour went through oklab and left the gamut,
which turned every pulse on both canvases fluorescent yellow.

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

frontend/src/components/Sidebar/AppSidebar.tsx

@@ -1,6 +1,7 @@
 import {
   Activity,
   Bell,
+  Brain,
   Home,
   KeyRound,
   LayoutDashboard,
@@ -26,6 +27,7 @@ const baseItems: Item[] = [
   // "Home" rather than "Dashboard": dashboards are their own thing now.
   { 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