feat(visualize): treemap infographic as the default view

A node-link graph of concept names is low-information — you see dots, not
content or structure. Replace the radial mind-map with an at-a-glance
treemap (what was asked for: an informative infographic, aggregated not
topological):
- each ingested document is a block (area ∝ its concepts' total
  connectivity); inside, its concepts are squarified tiles (tile area ∝
  connectivity, colour = type) → you see which documents are biggest, what
  each covers, and which concepts are the heavy/cross-cutting ones
- click a tile to read its details/sources/links; search filters tiles;
  legend filters types; reset clears
- DOM render (crisp text, no canvas legibility issues); a TREEMAP <-> 3D
  GRAPH toggle keeps the force layout for the visual overview
Removes the rejected radial mind-map.

Author: KylinMountain