Add unweighted shortest-path between two selected nodes

[enricobono99]

Add a "Shortest Path" action that, given exactly two selected nodes, adds the fewest-hops path connecting them - its nodes and the edges between them - to the current selection. Computed on the visible subgraph so it honours active filters.

• New pure, node-safe module graph/shortest_path.js (mirrors communities.js / visible_graph.js): buildVisibleGraph(cache) + graphology bidirectional BFS, returning {found, nodes, edges, hops}; edge ids derived from consecutive node pairs via graph.edges(u, v) on the undirected multigraph.
• Promote graphology-shortest-path to a direct dependency, export bidirectional from the graphology vendor entry, add it to the bundle pkgs list, and rebundle (graphology.bundle.mjs).
• selectShortestPathBetweenSelected() on GraphSelectionManager reuses the existing updateSelectedState selection machinery.
• "Shortest Path" button in the Select Elements card, below Expand/Reduce Neighbors, gated via toggleStyleElementsThatRequireExactlyTwoSelectedNodes (enabled only when exactly two nodes are selected).
• tests/shortest-path.test.js covers direct/multi-hop, shorter-of-two, undirected reverse, disconnected, source==target, non-visible endpoint, hidden edge, and parallel edges.