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.Storage.Sqlite/SqliteConfigStores.cs

@@ -34,6 +34,9 @@ public sealed class SqliteJoinTokenStore : IJoinTokenStore
 
     public ValueTask<bool> RevokeAsync(string id, long nowMs, CancellationToken ct) =>
         _db.WriteAsync(c => c.Exec("UPDATE join_tokens SET revoked_at = @now WHERE id = @id AND revoked_at IS NULL", ("@id", id), ("@now", nowMs)) == 1, ct);
+
+    public ValueTask<bool> DeleteAsync(string id, CancellationToken ct) =>
+        _db.WriteAsync(c => c.Exec("DELETE FROM join_tokens WHERE id = @id", ("@id", id)) == 1, ct);
 }
 
 public sealed class SqliteProbeStore : IProbeStore