[Themes] Route theme writes to the user themes directory

setColorScheme()/setStyleName() and the palette editor wrote directly to
the resolved theme directory, which for built-in themes is the read-only
system (install) location. Changes therefore landed in the install dir and
were lost on upgrade.

Add ThemeManager::writableThemeDir(), which always resolves to the user
themes directory, and route all theme writes through it. The palette editor
reuses the same helper, dropping its private writability probe.

cockatrice/src/interface/palette_editor/palette_editor_dialog.cpp

@@ -1,6 +1,5 @@
 #include "palette_editor_dialog.h"
 
-#include "../../client/settings/cache_settings.h"
 #include "../theme_manager.h"
 #include "palette_generator.h"
 #include "palette_grid_widget.h"
@@ -11,31 +10,11 @@
 #include <QDialogButtonBox>
 #include <QDir>
 #include <QFile>
-#include <QFileInfo>
 #include <QFrame>
-#include <QGuiApplication>
 #include <QLabel>
-#include <QLoggingCategory>
 #include <QMessageBox>
 #include <QPushButton>
-#include <QStyleHints>
 #include <QTimer>
-#include <libcockatrice/settings/paths_settings.h>
-
-// Probe whether a directory is truly writable by trying to create and remove a
-// temporary file. QFileInfo::isWritable() on a directory is unreliable (notably
-// on Windows where UAC VirtualStore can make a system dir appear writable).
-static bool isDirReallyWritable(const QString &dirPath)
-{
-    const QString probe = QDir(dirPath).absoluteFilePath(".cockatrice_write_test");
-    QFile f(probe);
-    if (!f.open(QIODevice::WriteOnly)) {
-        return false;
-    }
-    f.close();
-    f.remove();
-    return true;
-}
 
 PaletteEditorDialog::PaletteEditorDialog(const QString &_themeDirPath, const QString &_themeName, QWidget *parent)
     : QDialog(parent), themeDirPath(_themeDirPath), themeName(_themeName)
@@ -46,14 +25,7 @@ PaletteEditorDialog::PaletteEditorDialog(const QString &_themeDirPath, const QSt
     // Resolve a writable directory for saving. Built-in (Default / Fusion) and
     // other read-only theme directories must be customised in the user-writable
     // themes directory; otherwise the write would fail or be lost on upgrade.
-    if (!themeDirPath.isEmpty() && isDirReallyWritable(themeDirPath)) {
+    saveDir = ThemeManager::writableThemeDir(themeName);
-        saveDir = themeDirPath;
-    } else {
-        saveDir = QDir(SettingsCache::instance().paths().getThemesPath()).absoluteFilePath(themeName);
-        if (!QDir().mkpath(saveDir)) {
-            qWarning() << "Failed to create palette save directory:" << saveDir;
-        }
-    }
 
     // Load both scheme configs upfront so switching is instant
     loadSchemes();
@@ -214,7 +186,7 @@ void PaletteEditorDialog::retranslateUi()
     resetBtn->setToolTip(tr("Discard unsaved edits and restore the last saved palette"));
     saveBtn->setToolTip(tr("Write palette-%1.toml and reload the theme").arg(loadedScheme.toLower()));
 
-    if (saveDir.isEmpty() || !isDirReallyWritable(saveDir)) {
+    if (saveDir.isEmpty() || !ThemeManager::isDirReallyWritable(saveDir)) {
         saveBtn->setEnabled(false);
         saveBtn->setToolTip(tr("Cannot save: this theme has no writable directory"));
     }

cockatrice/src/interface/theme_manager.cpp

@@ -6,6 +6,7 @@
 #include <QApplication>
 #include <QColor>
 #include <QDebug>
+#include <QFile>
 #include <QFileInfo>
 #include <QLibraryInfo>
 #include <QMap>
@@ -184,11 +185,32 @@ QString ThemeManager::assetPath(QStringView prefix) const
     return resolvedPlain.isEmpty() ? prefix.toString() : resolvedPlain;
 }
 
-bool ThemeManager::isBuiltInTheme()
+// Probe whether a directory is truly writable by trying to create and remove a
+// temporary file. QFileInfo::isWritable() on a directory is unreliable (notably
+// on Windows where UAC VirtualStore can make a system dir appear writable).
+bool ThemeManager::isDirReallyWritable(const QString &dirPath)
 {
-    const auto themeName = SettingsCache::instance().getThemeName();
+    const QString probe = QDir(dirPath).absoluteFilePath(".cockatrice_write_test");
+    QFile f(probe);
+    if (!f.open(QIODevice::WriteOnly)) {
+        return false;
+    }
+    f.close();
+    f.remove();
+    return true;
+}
 
-    return themeName == NONE_THEME_NAME || themeName == FUSION_THEME_NAME;
+QString ThemeManager::writableThemeDir(const QString &themeName)
+{
+    // All theme writes go to the user themes directory regardless of whether
+    // the resolved (system) theme directory happens to be writable. Even when a
+    // write would succeed in-place, routing it to the user directory keeps the
+    // install intact and guarantees changes survive upgrades.
+    const QString dirPath = QDir(SettingsCache::instance().paths().getThemesPath()).absoluteFilePath(themeName);
+    if (!QDir().mkpath(dirPath)) {
+        qWarning() << "Failed to create theme save directory:" << dirPath;
+    }
+    return dirPath;
 }
 
 // System (read-only) themes location, relative to the application binary.
@@ -331,7 +353,7 @@ bool ThemeManager::commitPalette(const QString &themeDirPath, const QString &col
 
 void ThemeManager::setColorScheme(const QString &scheme)
 {
-    const QString dirPath = getAvailableThemes().value(SettingsCache::instance().getThemeName());
+    const QString dirPath = writableThemeDir(SettingsCache::instance().getThemeName());
     ThemeConfig cfg = ThemeConfig::fromThemeDir(dirPath);
 
     cfg.colorScheme = scheme;
@@ -342,7 +364,7 @@ void ThemeManager::setColorScheme(const QString &scheme)
 
 void ThemeManager::setStyleName(const QString &styleName)
 {
-    const QString dirPath = getAvailableThemes().value(SettingsCache::instance().getThemeName());
+    const QString dirPath = writableThemeDir(SettingsCache::instance().getThemeName());
     ThemeConfig cfg = ThemeConfig::fromThemeDir(dirPath);
 
     cfg.styleName = styleName;

cockatrice/src/interface/theme_manager.h

@@ -66,7 +66,16 @@ protected:
                               const QString &activeScheme);
 
 public:
-    bool isBuiltInTheme();
+    // Resolves the directory to write theme changes to for the given theme
+    // name. The resolved theme dir (user or system) is used when writable;
+    // read-only system themes fall back to the user themes directory, creating
+    // it if needed, so customisations never get lost on upgrade.
+    static QString writableThemeDir(const QString &themeName);
+    // Probe whether a directory is truly writable by trying to create and remove
+    // a temporary file. QFileInfo::isWritable() on a directory is unreliable
+    // (notably on Windows where UAC VirtualStore can make a system dir appear
+    // writable).
+    static bool isDirReallyWritable(const QString &dirPath);
     // Explicit color scheme of the theme: theme.cfg's ColorScheme setting
     // (Dark/Light), falling back to the OS color scheme when it is "System".
     bool isDarkMode(const QString &themeDirPath) const;