Add collapsible span tree rows

[goyalayus]

Context

The trace span tree gets hard to use when one parent span contains a large nested subtree. This shows up pretty quickly in agent traces: a top-level phase may contain many LLM, tool, and internal spans, and today the viewer forces every descendant row to stay visible.

This PR adds a small collapse affordance so users can hide noisy subtrees while keeping the overall trace timeline readable.

What changed

• Rows with child spans now render a chevron control.
• Collapsing a parent hides all descendant rows under it.
• Collapsed rows show a compact +N count for hidden descendants.
• Clicking the row still selects the span; clicking the chevron only expands/collapses the subtree.
• Deep links and URL-selected spans expand their ancestors before scrolling, so hidden targets still work.
• The span-tree bookkeeping is kept in small pure helpers (buildSpanTreeModel, getVisibleSpanRows, getAncestorIds) instead of pushing more branching into the render path.

Why this shape

I kept the feature local to SpanTree because this is a UI-only affordance: it does not change trace data, span ordering, annotations, or the timeline math. The helper functions make the collapse behavior easier to reason about and should leave room for future trace-tree features like expand-all, collapse-all, filtering, or search without turning the component into a pile of nested state logic.

Verification

• bun x eslint app/src/components/SpanTree.tsx
• cd app && bun x vite build