feat: show only used disk space in server monitoring chart

[lasseveenliese]

What is this PR about?

This PR updates the Disk Space monitoring chart to show only used storage and aligns its styling with the other gray/black charts.

Issues related

closes #3946

Greptile Summary

This PR simplifies the Docker disk space monitoring chart by displaying only used disk space (removing the "Free GB" area series), and realigns the chart's visual styling — gradient fill, stroke, and grid colors — from purple (#6C28D9) to the gray/dark palette (#27272A) already used by the other monitoring charts (CPU, memory, network). The Legend component and the freeGb data field are also removed throughout. The tooltip retains the "Total disk" line for reference, keeping enough context for the user.

Changes:

• Removes the Free GB <Area> series and its associated colorFree gradient definition
• Renames the colorUsed gradient to colorDisk and updates colours to match the gray/dark theme
• Drops freeGb from the data transform and tooltip payload
• Removes the <Legend /> component (import cleaned up accordingly)

Confidence Score: 5/5

• This PR is safe to merge — it is a minimal, focused UI-only change with no logic regressions.
• The change removes a chart series and updates styling colours. It is consistent with patterns already used by sibling charts (CPU, memory) in the same directory. The totalGb value is still passed through for the tooltip, and the Y-axis domain ([0, diskTotal]) continues to rely on the prop correctly. No new gradient ID collisions were found — colorDisk is only defined in this single component, which is instantiated once per page. No TypeScript errors are introduced.
• No files require special attention.

_{Last reviewed commit: fc8a515}

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