feat(vdisplay): display-management policy surface (Stage 0)

A user-configurable policy layer above the per-compositor VirtualDisplay
backends: keep-alive, topology, conflict, identity, layout, max-displays —
persisted to display-settings.json, editable from the web console, applied
per connect. Design: design/display-management.md.

Stage 0 stands up the surface and wires the two behaviors the existing code
can already express — the Windows monitor linger duration and the
"make the streamed output the sole desktop" topology — through it; every
other option is stored + echoed but not yet enforced (later stages). An
unconfigured host (no display-settings.json) keeps today's exact behavior.

- vdisplay/policy.rs: pure DisplayPolicy + 5 presets + JSON store (gpu-settings
  pattern) + EffectivePolicy; 9 unit tests.
- vdisplay.rs: resolve_topology(Auto); apply_session_env drives *_VIRTUAL_PRIMARY
  from the policy only when a settings file exists.
- windows/manager.rs: linger_ms() + should_isolate() read the policy when configured.
- mgmt: GET/PUT /api/v1/display/settings (bearer-only); PUT rejects keep_alive
  forever until the lifecycle stage. OpenAPI regenerated.
- web console: Host → Virtual displays card (preset picker + custom fields); en+de.
- docs-site: virtual-displays.md + configuration.md cross-links.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/sections/Host/index.tsx

@@ -1,6 +1,7 @@
 import type { FC } from "react";
 import { useGetHostInfo, useListCompositors } from "@/api/gen/host/host";
 import { useLocale } from "@/lib/i18n";
+import { DisplaySection } from "./DisplayCard";
 import { GpuSection } from "./GpuCard";
 import { HostView } from "./view";
 
@@ -10,6 +11,11 @@ export const SectionHost: FC = () => {
 	const compositors = useListCompositors();
 
 	return (
-		<HostView host={host} compositors={compositors} gpu={<GpuSection />} />
+		<HostView
+			host={host}
+			compositors={compositors}
+			gpu={<GpuSection />}
+			displays={<DisplaySection />}
+		/>
 	);
 };