[App] Add onboarding wizard (#7064)

* [App] Add onboarding wizard

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* Adjust CI

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* Adjust CI again

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* Comments and fixes

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* Rebase.

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* Comments.

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* CI lol

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* CI again lol

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* Drop some settings, add some new ones.

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* Resize when expanding section

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Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/widgets/onboarding/pages/theme_setup_page.h

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+#ifndef THEME_SETUP_PAGE_H
+#define THEME_SETUP_PAGE_H
+
+#include "../first_run_wizard_page.h"
+
+class QComboBox;
+class QGroupBox;
+class QuickSetupPanel;
+
+/** @brief First-run theme step. Reuses the same building blocks as Appearance
+ *         settings and the Palette Editor (ThemeManager, PaletteConfig,
+ *         PaletteGenerator, and the QuickSetupPanel widget itself) rather than
+ *         reimplementing palette generation or preview here.
+ *
+ * Behavior specific to this page (deliberately not pushed down into
+ * ThemeManager, to avoid changing app-wide behaviour for existing installs):
+ *  - Opening the page never changes the running palette; previews and
+ *    auto-generation only happen when the user actually changes a control.
+ *  - If a theme+scheme the user selects has no saved palette and no shipped
+ *    default, one is generated from the QuickSetupPanel's current accent so
+ *    the preview doesn't fall back to a flat, unstyled look. */
+class ThemeSetupPage : public FirstRunWizardPage
+{
+    Q_OBJECT
+
+public:
+    explicit ThemeSetupPage(QWidget *parent = nullptr);
+
+    void initializePage() override;
+    bool validatePage() override;
+    bool isSkippable() const override;
+    QString stepTitle() const override;
+    QString stepSubtitle() const override;
+    void retranslateUi() override;
+
+private slots:
+    void onThemeChanged(int index);
+    void onSchemeChanged();
+    void onGenerateFromAccent(const QColor &accent, int intensity);
+    void onHomeTabBackgroundChanged(int index);
+
+private:
+    QString currentScheme() const;
+    QString resolvedScheme() const; // "System" -> actual Light/Dark
+    void maybeAutoGeneratePalette();
+    QString writableThemeDir() const;
+
+    QComboBox *themeCombo;
+    QComboBox *schemeCombo;
+    QGroupBox *accentGroup;
+    QuickSetupPanel *quickSetupPanel;
+
+    QComboBox *homeTabBackgroundCombo;
+
+    bool paletteDirty = false;
+};
+
+#endif // THEME_SETUP_PAGE_H