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/lib/labels.ts

@@ -31,12 +31,6 @@ export const STACK_LABELS: Record<string, string> = {
   remote: "Remote",
 }
 
-export const SECTION_HEADERS: Record<string, { title: string; subtitle: string }> = {
-  service: { title: "Services", subtitle: "Long-running processes" },
-  scheduled: { title: "Scheduled Jobs", subtitle: "Systemd timers" },
-  program: { title: "Programs", subtitle: "Software catalog" },
-}
-
 export function runnerLabel(runner: string): string {
   return RUNNER_LABELS[runner] ?? runner
 }