Persist waterfall color scheme selection across sessions

[AetherClaude]

Summary

• The waterfall heat map color gradient was hard-coded (black/blue/cyan/green/yellow/red) with no way to change it and no persistence
• Adds five selectable waterfall color schemes: Default, Grayscale, Blue-Green, Fire, Plasma
• Selection is persisted via AppSettings (DisplayWfColorScheme key) and restored on launch
• Per-pan support: each panadapter can have its own color scheme

Changes

• src/gui/SpectrumWidget.h -- Added WfColorScheme enum, WfGradientStop struct, helper functions (wfSchemeStops, wfSchemeName), new member m_wfColorScheme, setter/getter
• src/gui/SpectrumWidget.cpp -- Defined five gradient presets as constexpr arrays, added shared interpolateGradient() helper, replaced hard-coded gradients in dbmToRgb() and intensityToRgb() with scheme-aware lookup, added setWfColorScheme() setter with AppSettings persistence, load in loadSettings()
• src/gui/SpectrumOverlayMenu.h -- Added m_colorSchemeCmb member, wfColorSchemeChanged(int) signal, extended syncDisplaySettings() with colorScheme parameter
• src/gui/SpectrumOverlayMenu.cpp -- Added combo box in waterfall section of display panel, sync logic in syncDisplaySettings()
• src/gui/MainWindow.cpp -- Wired wfColorSchemeChanged signal from overlay menu to SpectrumWidget::setWfColorScheme

Testing

• Launch AetherSDR, open the spectrum overlay display panel
• Change the "Scheme" combo box to any non-default option (e.g. Fire)
• Confirm the waterfall updates visually to reflect the new palette
• Quit and relaunch -- verify the waterfall uses the previously selected scheme
• In multi-pan mode, verify each pan can have an independent color scheme
• Inspect ~/.config/AetherSDR/AetherSDR.settings for the DisplayWfColorScheme key

Fixes aethersdr#733

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Generated by AetherClaude (automated agent)