Add app shell with left nav; split dashboard into top-level pages

Introduce a Layout shell with a collapsible left sidebar (state persisted
to localStorage) that becomes a hamburger drawer under md. Break the single
Dashboard into routed pages — Overview (summary tiles + node bar), Services,
Scheduled, Programs, Gateway, Mesh — nested under the shared Layout via an
Outlet. Centralize useEventStream in Layout (drop the per-page duplicates),
repoint detail-page back links at their section pages, and remove the now-dead
SectionHeader component / SECTION_HEADERS map (each page owns its header).

app/src/components/ServiceSection.tsx

@@ -1,7 +1,6 @@
 import { useMemo } from "react"
 import type { ServiceSummary, HealthStatus } from "@/types"
 import { ServiceCard } from "./ServiceCard"
-import { SectionHeader } from "./SectionHeader"
 
 interface ServiceSectionProps {
   services: ServiceSummary[]
@@ -12,13 +11,10 @@ export function ServiceSection({ services, statuses }: ServiceSectionProps) {
   const statusMap = useMemo(() => new Map(statuses.map((s) => [s.id, s])), [statuses])
 
   return (
-    <section>
-      <SectionHeader section="service" />
-      <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
-        {services.map((svc) => (
-          <ServiceCard key={svc.id} service={svc} health={statusMap.get(svc.id)} />
-        ))}
-      </div>
-    </section>
+    <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
+      {services.map((svc) => (
+        <ServiceCard key={svc.id} service={svc} health={statusMap.get(svc.id)} />
+      ))}
+    </div>
   )
 }