Add SVG export context menu for MIES graphs

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This update focuses on the SVG export context menu for MIES plots and hardens the export path/filename handling so the new option is reliable across supported windows.

• Menu integration: add a global GraphPopup entry that targets the right-clicked graph window.
• Path resolution: resolve Windows Downloads via PowerShell → HFS conversion with diagnostics and a Documents fallback.
• Export behavior: sanitize filenames, add local-time timestamps, and surface SavePICT failures with full context.

Menu "GraphPopup"
	"Export graph to SVG", /Q, ExportGraphToSVG(GetCurrentWindow())
End

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[t-b]

Closing as duplicate of #2630