Viewer: dark theme, switchable live from the View menu

View > Theme offers Follow system / Light / Dark, stored in the settings.
Follow system uses the desktop's colour scheme where Qt reports it (6.5+)
and is light otherwise. The dark theme is a deep indigo (#1A1D3A panels,
#12142B entry fields and charts).

The palettes live in the new ViewerTheme module. A switch applies at once:
the application stylesheet is set again after the palette so every widget
is re-polished (with a stylesheet active, polished widgets otherwise keep
their old palette); widgets that bake colours into stylesheets use
SetThemedStyleSheet, which re-applies them on ThemeNotifier::changed;
toolbar icons draw their glyph at paint time through an icon engine, so the
ink follows the theme (and stays sharp at any size); charts re-theme and
rebuild. Hard-coded white backgrounds on entry fields and tables are
removed in favour of the palette's base colour.

The idle detector-status badge in the status bar is now transparent instead
of an empty default-styled progress bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

viewer/widgets/SliderPlusBox.cpp

@@ -33,7 +33,6 @@ SliderPlusBox::SliderPlusBox(double min, double max, double step, int decimals,
     m_doubleSpinBox->setDecimals(decimals);
     m_doubleSpinBox->setSingleStep(step);
     m_doubleSpinBox->setValue(v);
-    m_doubleSpinBox->setStyleSheet("background-color: rgb(255, 255, 255);");
 
     m_updateTimer = new QTimer(this);
     m_updateTimer->setInterval(500);  // 500ms throttle