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.