feat: node info, build info and acrylic web theme

Squash of recent dev work: return_session login, backend switcher, line
detail timeline, overview strips, node platform/country/ISP info with flags
and icons, build info (version, tag, time) behind site.show_build, and the
acrylic display theme.

backend/src/Monitor.Backend.Application/Agents/AgentService.cs

@@ -20,8 +20,11 @@ public sealed partial class AgentService
     private readonly TaskPlanner _planner;
     private readonly ISettingsStore _settings;
 
-    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings)
+    private readonly NodeInfoService _nodes;
+
+    public AgentService(IAgentStore agents, AgentRuntime runtime, IAgentGateway gateway, TaskPlanner planner, ISettingsStore settings, NodeInfoService nodes)
     {
+        _nodes = nodes;
         _agents = agents;
         _runtime = runtime;
         _gateway = gateway;
@@ -114,6 +117,7 @@ public sealed partial class AgentService
     {
         await _gateway.DisconnectAsync(id, "deleted", ct);
         await _agents.DeleteAsync(id, ct);
+        await _nodes.ForgetAsync(id.Value, ct);
         _runtime.Remove(id);
         await AfterAgentSetChangedAsync(ct);
     }