[Feature] CSV Export for User-Facing History Charts

## Problem Statement

CSV export already exists for the internal dev-only Render Metrics overlay (Ctrl+Shift+M, `src/UI/RenderMetrics.{h,cpp}`), but not for the actual CPU/memory/network/GPU history charts users look at in System Overview or Process Details.

## Proposed Solution

An export action on each history chart (or a panel-level "Export history" action) writing the currently-plotted history buffer to CSV.

## Alternatives Considered

JSON instead of, or alongside, CSV — CSV is the more universally-consumable format for this use case (spreadsheet import), so it's the reasonable starting point.

## Additional Context

n/a

## Implementation Notes

`src/UI/ChartWidgets.h`'s shared `HistoryChart` RAII helper is the single place every history chart already renders through — export logic belongs there so it applies uniformly across every chart rather than being re-implemented per panel.

Start in src/UI/ChartWidgets.h and inspect the shared HistoryChart helper, then use src/UI/RenderMetrics.h and src/UI/RenderMetrics.cpp as the existing CSV-export reference. Add an export action for the currently plotted history buffer and verify it works consistently for the CPU, memory, network, and GPU charts in System Overview and Process Details.

Written by the indexing model from the issue text.