WIP [UI] Theme-aware onboarding banner with frosted light mode

Banner colours now derive from palette tokens at ~60fps (tick-driven,
equality-guarded setters) so scheme switches and live accent-picker
previews apply instantly:

- dark stages: byte-for-byte the original treatment (near-black stage
  from window hue, Highlight accent, white centre halo, vignette 0.62)
- light stages: pastel accent-hue wash instead of a neutral grey copy,
  brightness-lifted accent for additive glow legibility, deep-Highlight
  halo (uGlowColor) instead of white blowout, gentler vignette
  (uVignetteMin 0.88) so corners don't go muddy
- black logo silhouette variant selected on light stages
- theme picker preseeded with brand green (brand_colors.h single source)

WIP notes for next session:
- real-pixel wizard screenshot check still pending (headless capture
  exists: Xvfb :77 + isolated XDG_DATA_HOME; shader vs fallback pixel
  analysis not finished)
- user plans separately: promote Fusion to default theme, Default -> system

cockatrice/src/interface/widgets/onboarding/shader_banner_widget.cpp

@@ -1,7 +1,10 @@
 #include "shader_banner_widget.h"
 
+#include "../../theme_manager.h"
 #include "banner_shader_config.h"
+#include "brand_colors.h"
 
+#include <QApplication>
 #include <QPainter>
 #include <QQmlContext>
 #include <QQmlEngine>
@@ -11,11 +14,68 @@
 
 namespace
 {
-// Near-black base palette -- the background is dark and quiet so the green
-// accent stands out.
-constexpr QRgb kColorA = 0x1A1A20;
-constexpr QRgb kColorB = 0x0E0E12;
-constexpr QRgb kAccent = 0x8BDD6B;
+// Curated near-black stage -- used only when the active palette resolves no
+// usable window colour. Matches the banner's original design (dark and quiet
+// so the accent stands out) and satisfies design-plans §2.1's "identity
+// survives a bare palette".
+constexpr QRgb kFallbackColorA = 0x1A1A20;
+constexpr QRgb kFallbackColorB = 0x0E0E12;
+
+struct SuggestedColors
+{
+    QColor colorA;
+    QColor colorB;
+    QColor accent;
+    QColor glowColor;
+    qreal vignetteMin = 0.62;
+    bool lightStage = false;
+};
+
+SuggestedColors suggestedBannerColors()
+{
+    const QPalette &pal = qApp->palette();
+    const QColor window = pal.color(QPalette::Active, QPalette::Window);
+    const QColor highlight = pal.color(QPalette::Active, QPalette::Highlight);
+    if (!window.isValid() || !highlight.isValid()) {
+        return {
+            QColor(kFallbackColorA), QColor(kFallbackColorB), kCockatriceBrandGreen, QColor(Qt::white), 0.62, false};
+    }
+
+    // Dress the stage for the scheme so the banner never fights the
+    // surrounding window in either mode. Dark palettes keep the original
+    // quiet near-black stage (lightness 29 → 16) with the theme's window
+    // hue; light palettes get a pastel "frosted accent" treatment built from
+    // the Highlight hue instead of a plain near-white copy: a gentle mint
+    // wash that clearly belongs to the theme.
+    const qreal luma = 0.299 * window.red() + 0.587 * window.green() + 0.114 * window.blue();
+    const bool lightStage = luma > 115.0;
+    if (lightStage) {
+        const int hue = highlight.hslHue();
+        // Achromatic accents (grey) get a neutral near-white stage instead.
+        const int stageSat = hue < 0 ? 0 : 35;
+        const int hueSafe = hue < 0 ? 0 : hue;
+        auto pastel = [hueSafe, stageSat](int lightness) { return QColor::fromHsl(hueSafe, stageSat, lightness); };
+        // Brightness-lifted accent for additive glows: Highlight on a light
+        // stage must be mid-bright to read (the shipped light Highlight is a
+        // deep green that washes out additively against white).
+        const int accentLightness = qBound(120, highlight.lightness() + 70, 165);
+        const int accentSaturation = hue < 0 ? 0 : qMax(highlight.hslSaturation(), 140);
+        const QColor liftedAccent = hue < 0 ? highlight : QColor::fromHsl(hueSafe, accentSaturation, accentLightness);
+        // The centre glow uses the deep Highlight itself -- a coloured halo
+        // behind the dark logo instead of a white blowout.
+        return {pastel(247), pastel(231), liftedAccent, highlight, 0.88, true};
+    }
+
+    // Dark stage: force the window hue down to the banner's curated darkness,
+    // scaling saturation away so chromatic palettes tint it without going
+    // muddy. White glow and the original strong vignette stay untouched.
+    auto stage = [&window](int lightness) {
+        const int hue = window.hslHue();
+        const int saturation = hue < 0 ? 0 : qBound(0, qRound(window.hslSaturation() * (lightness / 40.0)), 255);
+        return QColor::fromHsl(hue, saturation, lightness);
+    };
+    return {stage(29), stage(16), QColor(highlight), QColor(Qt::white), 0.62, false};
+}
 } // namespace
 
 class GradientFallbackWidget : public QWidget
@@ -23,15 +83,27 @@ class GradientFallbackWidget : public QWidget
 public:
     using QWidget::QWidget;
 
+    void setColors(const QColor &a, const QColor &b)
+    {
+        if (a != colorA || b != colorB) {
+            colorA = a;
+            colorB = b;
+        }
+    }
+
 protected:
     void paintEvent(QPaintEvent *) override
     {
         QPainter painter(this);
         QLinearGradient gradient(0, 0, width(), height());
-        gradient.setColorAt(0.0, QColor(kColorA));
-        gradient.setColorAt(1.0, QColor(kColorB));
+        gradient.setColorAt(0.0, colorA);
+        gradient.setColorAt(1.0, colorB);
         painter.fillRect(rect(), gradient);
     }
+
+private:
+    QColor colorA{QColor(kFallbackColorA)};
+    QColor colorB{QColor(kFallbackColorB)};
 };
 
 BannerHost::BannerHost(QWidget *parent) : QWidget(parent)
@@ -62,6 +134,9 @@ BannerHost::BannerHost(QWidget *parent) : QWidget(parent)
     connect(&clock, &QTimer::timeout, this, &BannerHost::tick);
     clock.setInterval(16); // ~60fps; the shader itself is cheap, this is just a wall clock
 
+    connect(themeManager, &ThemeManager::themeChanged, this, &BannerHost::applyThemeColors);
+    applyThemeColors();
+
     applyMotifPreset(currentMotif);
     updateAspect();
 }
@@ -123,9 +198,9 @@ void BannerHost::applyMotifPreset(Motif motif)
 
     const Preset p = presetFor(motif);
 
-    config->setColorA(QColor(kColorA));
-    config->setColorB(QColor(kColorB));
-    config->setAccent(QColor(kAccent));
+    config->setColorA(bannerColorA);
+    config->setColorB(bannerColorB);
+    config->setAccent(bannerAccent);
     config->setLogoVisible(motif == Motif::Welcome);
 
     if (isFirstApply) {
@@ -183,8 +258,33 @@ void BannerHost::hideEvent(QHideEvent *event)
     clock.stop();
 }
 
+void BannerHost::applyThemeColors()
+{
+    const SuggestedColors colors = suggestedBannerColors();
+    bannerColorA = colors.colorA;
+    bannerColorB = colors.colorB;
+    bannerAccent = colors.accent;
+
+    if (usingFallback) {
+        fallback->setColors(bannerColorA, bannerColorB);
+        fallback->update();
+    } else if (config) {
+        config->setColorA(bannerColorA);
+        config->setColorB(bannerColorB);
+        config->setAccent(bannerAccent);
+        config->setGlowColor(colors.glowColor);
+        config->setVignetteMin(colors.vignetteMin);
+        config->setLogoDark(colors.lightStage);
+    }
+}
+
 void BannerHost::tick()
 {
+    // Palette previews (e.g. accent drags in the wizard's QuickSetupPanel)
+    // apply qApp->palette() without firing themeChanged, so re-derive here;
+    // BannerShaderConfig's setters are equality-guarded, so this is a no-op
+    // unless the colours actually changed.
+    applyThemeColors();
     if (config) {
         qreal t = elapsed.elapsed() / 1000.0;
         config->setTime(t);