Follow mode, expand/collapse all, filtering, style tweaks.

Features

• Follow mode now tracks sub-agent spans correctly (end-time-based).
• Explicit follow toggle checkbox in the Spans header.
• Collapse-all / expand-all buttons.
• Selecting an agent span routes ChatDetail to its most recent chat.
• Kind filter visibly dims non-matching rows.
• Spans header redesigned as a two-row grid layout.
• Context limit line labeled and thickened for clarity.

Fixes

• Kind filter was visually broken (.dim class didn't cascade through child elements).
• Default column title corrected from "Traces" to "Spans".