Task: Add X/Y axis labels to GMM preview charts

The Live Return Distribution Preview and Live Sample Price Paths Preview charts on the FX Rate → Price behaviour tab render without numeric axis labels/ticks, making them hard to read (see reference screenshot attached to the story). Add labelled X and Y axes to both charts.

• Live Return Distribution Preview shows a labelled X axis ("Return per Update %") and Y axis ("Probability") with numeric ticks.
• Live Sample Price Paths Preview shows a labelled X axis ("Update Steps") and Y axis ("Price") with numeric ticks.

Both charts (ReturnDistributionChart, SamplePricePathsChart in projects/ores.qt/synthetic/) already configured QValueAxis titles, ranges and grid/label colours in their constructors — the axes were never actually missing. The bug: both called palette().color(QPalette::WindowText) to pick text/grid colours, but the app's dark theme is applied purely via a QSS stylesheet (main.cpp, :/TradingStyle.qss), never via QApplication::setPalette, so QWidget::palette() on a freshly-constructed, unparented widget returns Qt's default light-mode colours — near-black text on QChart::ChartThemeDark's near-black background, effectively invisible. Fixed by switching both charts to the same explicit theme-colour constants FxSpotChartWindow already uses (QColor(0xCB, 0xD5, 0xE1) text, QColor(255, 255, 255, 18) grid) instead of reading palette().

Root cause confirmed by comparing against the working FxSpotChartWindow (projects/ores.qt/mktdata/), which hardcodes its axis colours rather than reading palette().

Fixed both ReturnDistributionChart and SamplePricePathsChart to use explicit theme colours instead of QWidget::palette(), which was returning invisible near-black text/gridlines against the dark chart background. Axis titles, ranges and ticks were already correctly configured; only the colour was wrong. Built ores.qt.synthetic.lib clean (clang debug/make).