Compare traces, split workspace & span panel

The trace explorer moved to a split layout: the trace list stays on the left while the detail opens on the right, so hopping between traces is one click. Clicking a span now opens a side panel (no more inline expand), and a maximize button pops the whole thing to a full-window view.

• Trace comparison (structural diff). Pin a second trace and see the two span trees aligned — added, removed, faster and slower spans are highlighted so you can debug "fast vs slow" across services at a glance.
• Split workspace. Collapsible trace-list rail ↔ trace detail; the selected trace, view, span and compare target all live in the URL (shareable).
• Span detail drawer. Attributes, events and status render in a dedicated side panel next to the waterfall.
• Themed filter dropdowns. The Status/Order selectors now match the app theme instead of the browser's default popup.