Add erlang process information (with simple chart)

[kraleppa]

Expose key Erlang process metrics for the LiveView process and visualize them in a lightweight, non-persistent chart in the Performance tab.

This task is result of #464
Consider using this library for chart https://github.com/Stax3/live_charts

Scope

• Collect per-LiveView process metrics:
  • reductions
  • memory
  • total_heap_size
  • heap_size
  • stack_size
  • gc_min_heap_size
  • gc_fullsweep_after
• Sample metrics at a small, fixed interval while the Performance tab is open (stop when closed).
• Render a simple in-browser chart (single series at a time with a dropdown to pick metric).
• Reset data on tab refresh; no persistence.

Technical Notes

• Use the PoC module from PR PoC: performance debugging techniques #786 as a base for gathering process data.
• Follow the “non-persistent charts” guidance; keep CPU/alloc overhead minimal.
• Ensure the target process is the LiveView’s owning process (not the transport process).

Acceptance Criteria

• Opening a LiveView’s Performance tab shows current metric values and a live-updating chart for the selected metric.
• Closing the tab halts sampling; reopening starts a fresh, empty series.
• Verified values match :erlang.process_info/2 (spot-checked) and update smoothly under load.
• No noticeable slowdown of the LiveView while the chart runs.

Out of Scope

• Persisting history, exporting data, multi-process comparisons.