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/ProbeService.cs

@@ -35,9 +35,11 @@ public sealed partial class ProbeService
     private readonly AgentRuntime _runtime;
     private readonly IAgentGateway _gateway;
     private readonly TimeProvider _time;
+    private readonly NodeInfoService _nodes;
 
-    public ProbeService(IProbeStore probes, IAgentStore agents, AgentService agentService, AgentRuntime runtime, IAgentGateway gateway, TimeProvider time)
+    public ProbeService(IProbeStore probes, IAgentStore agents, AgentService agentService, AgentRuntime runtime, IAgentGateway gateway, TimeProvider time, NodeInfoService nodes)
     {
+        _nodes = nodes;
         _probes = probes;
         _agents = agents;
         _agentService = agentService;
@@ -138,6 +140,7 @@ public sealed partial class ProbeService
         bool deleted = await _probes.DeleteAsync(id, ct);
         if (deleted)
         {
+            await _nodes.ForgetAsync(id, ct);
             _gateway.NotifyAllTaskSetsChanged();
         }