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/messages/en.json

@@ -47,6 +47,27 @@
 	"gpu_none": "No GPUs detected.",
 	"gpu_missing_warning": "The preferred GPU “{name}” is not present — automatic selection is used instead.",
 	"gpu_env_note": "PUNKTFUNK_RENDER_ADAPTER={value} pins the GPU while in automatic mode.",
+	"host_displays": "Virtual displays",
+	"host_displays_help": "How virtual displays are created, kept alive, and arranged. Pick a preset, or choose Custom to set options directly. A change applies to the next session.",
+	"display_preset": "Preset",
+	"display_preset_custom": "Custom",
+	"display_preset_default": "Default",
+	"display_preset_gaming_rig": "Gaming rig",
+	"display_preset_shared_desktop": "Shared desktop",
+	"display_preset_hotdesk": "Hot-desk",
+	"display_preset_workstation": "Workstation",
+	"display_keep_alive": "Keep alive after disconnect",
+	"display_keep_alive_off": "Off",
+	"display_keep_alive_seconds": "seconds",
+	"display_topology": "Topology",
+	"display_topology_auto": "Automatic",
+	"display_topology_extend": "Extend",
+	"display_topology_primary": "Primary",
+	"display_topology_exclusive": "Exclusive",
+	"display_max": "Max displays",
+	"display_save": "Save",
+	"display_effective": "In effect",
+	"display_pending_note": "Conflict handling, identity, and layout are stored but not enforced yet — they arrive in later releases.",
 	"clients_title": "Paired clients",
 	"clients_empty": "No paired clients yet.",
 	"clients_name": "Name",