Calculate assigns size (node memory usage) in node inspector

[kraleppa]

Display the total memory size of all assigns for the current LiveView directly in the Node Inspector.

This task is result of #464

Scope

• Compute and show cumulative memory footprint of assigns next to the assigns list in Node Inspector.
• Recalculate on state changes
• Indicate unit (bytes/KB/MB) with smart formatting.
• Leverage earlier partial work referenced in PR Added liveview memory usage + estimate of the assigns #312.

Acceptance Criteria

• Node Inspector shows a “Assigns size: X KB/MB” value that updates when assigns change.
• Size is stable (± small variance) across repeated openings.
• On a synthetic large-assigns demo, value increases/decreases as expected when adding/removing data.
• No measurable degradation of render latency.

Out of Scope

• Per-key breakdown UI or historical charts.