[web-shared] stabilize detail panel rendering during polling, add loading skeletons, and improve error display

[karthikscale3]

Description:

What changed

• SelectionBridge now tracks the previous spanId and skips re-notifying the parent when the trace rebuilds during polling but the selection hasn't actually changed
• EntityDetailPanel passes isLoading to EventsList so the events section shows skeletons while span details are being fetched
• AttributePanel renders a resumeAt skeleton row inside the basic attributes table for sleep spans during loading
• EventItem error state uses ErrorCard instead of a plain styled div

Why

• On live runs, every poll cycle rebuilt the trace and gave selected a new reference, causing SelectionBridge to create a new selectedSpan object and re-render the entire detail panel (input/output cards, JSON blocks) every ~5 seconds
• Selecting a span triggered a detail fetch but showed no loading indication for the events section or the resumeAt field — the content just popped in after the fetch completed
• Expanding an event that failed to load showed a raw unstyled error string ("failed to fetch") instead of using the existing ErrorCard component