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/Ports/StorePorts.cs

@@ -33,6 +33,22 @@ public interface IAgentStore
     ValueTask SetAppliedTaskStateAsync(NodeId id, long appliedVersion, string? rejectedJson, CancellationToken ct);
 }
 
+/// <summary>节点的展示信息（系统、架构、国家、运营商）。</summary>
+public interface INodeMetaStore
+{
+    ValueTask<IReadOnlyList<NodeMeta>> ListAsync(CancellationToken ct);
+
+    ValueTask SaveAsync(NodeMeta meta, CancellationToken ct);
+
+    ValueTask DeleteAsync(string nodeId, CancellationToken ct);
+}
+
+/// <summary>查一个 IP（或域名）的归属：国家、ASN、组织。实现负责超时和失败（失败返回 null，不抛）。</summary>
+public interface IGeoLookup
+{
+    ValueTask<GeoInfo?> LookupAsync(string host, CancellationToken ct);
+}
+
 public interface IJoinTokenStore
 {
     ValueTask<JoinTokenInfo> CreateAsync(string id, byte[] tokenSha256, string? note, IReadOnlyList<string> labels, bool autoApprove, int maxUses, long expiresAtMs, long nowMs, CancellationToken ct);
@@ -40,6 +56,8 @@ public interface IJoinTokenStore
     ValueTask<IReadOnlyList<JoinTokenInfo>> ListAsync(CancellationToken ct);
 
     ValueTask<bool> RevokeAsync(string id, long nowMs, CancellationToken ct);
+
+    ValueTask<bool> DeleteAsync(string id, CancellationToken ct);
 }
 
 public interface IProbeStore