Fit-to-screen button uses hardcoded scale instead of computing bounds #10

Problem

`internal/web/static/index.html` lines ~1565-1570: The `fitGraph()` function applies `scale(0.8)` centered on the container midpoint instead of computing the actual bounding box of all nodes.

Impact

"Fit to screen" doesn't actually fit the graph. Nodes may be off-screen after fitting.

Fix

Compute min/max x,y across all nodes, calculate proper scale and translate to fit all content with padding.