Fix knowledge graph clustering — nodes now spread to fill viewport
Two root causes: (1) container height:100% resolved to auto because
parent .workspace has no explicit height, collapsing the flex layout
to near-zero height; fixed with viewport-based calc. (2) Physics
parameters too conservative for 50-150 node graphs — increased
repulsion, spring length, spring constant, and stabilization iterations.
Added explicit fit() after stabilization for insurance.
Fixes support ticket #3.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>