Append a graph to another graph

If the user open a graph and then a second one, the tool should propose to "append" the nodes and edges of the 2nd graph to those of the 1st instead of opening a new workspace (in the same way in which Gephi Desktop does).

Start by tracing how the web app opens a graph and creates a workspace. Determine where a second graph could be offered as an append operation, preserving the first graph while adding the second graph's nodes and edges. Done means opening two graphs presents the append choice and produces one combined graph, matching the stated Gephi Desktop behavior.

Written by the indexing model from the issue text.