Add theme system with Reclass Dark and Warm built-in themes

Replaces ~40 hardcoded color values with 27 semantic color roles.
Adds ThemeManager singleton, theme editor dialog, View > Theme menu,
JSON persistence for user themes, and fixes inline edit selection
color from blue #264f78 to #2b2b2b.

src/themes/thememanager.cpp

@@ -0,0 +1,119 @@
+#include "thememanager.h"
+#include <QSettings>
+#include <QDir>
+#include <QFile>
+#include <QJsonDocument>
+#include <QStandardPaths>
+
+namespace rcx {
+
+ThemeManager& ThemeManager::instance() {
+    static ThemeManager s;
+    return s;
+}
+
+ThemeManager::ThemeManager() {
+    m_builtIn.append(Theme::reclassDark());
+    m_builtIn.append(Theme::warm());
+    loadUserThemes();
+
+    QSettings settings("ReclassX", "ReclassX");
+    QString saved = settings.value("theme", m_builtIn[0].name).toString();
+    auto all = themes();
+    for (int i = 0; i < all.size(); i++) {
+        if (all[i].name == saved) { m_currentIdx = i; break; }
+    }
+}
+
+QVector<Theme> ThemeManager::themes() const {
+    QVector<Theme> all = m_builtIn;
+    all.append(m_user);
+    return all;
+}
+
+const Theme& ThemeManager::current() const {
+    if (m_currentIdx < m_builtIn.size())
+        return m_builtIn[m_currentIdx];
+    int userIdx = m_currentIdx - m_builtIn.size();
+    if (userIdx >= 0 && userIdx < m_user.size())
+        return m_user[userIdx];
+    return m_builtIn[0];
+}
+
+void ThemeManager::setCurrent(int index) {
+    auto all = themes();
+    if (index < 0 || index >= all.size()) return;
+    m_currentIdx = index;
+    QSettings settings("ReclassX", "ReclassX");
+    settings.setValue("theme", all[index].name);
+    emit themeChanged(current());
+}
+
+void ThemeManager::addTheme(const Theme& theme) {
+    m_user.append(theme);
+    saveUserThemes();
+}
+
+void ThemeManager::updateTheme(int index, const Theme& theme) {
+    if (index < builtInCount()) {
+        // Can't overwrite built-in; save as user theme instead
+        m_user.append(theme);
+    } else {
+        int ui = index - builtInCount();
+        if (ui >= 0 && ui < m_user.size())
+            m_user[ui] = theme;
+    }
+    saveUserThemes();
+    if (index == m_currentIdx)
+        emit themeChanged(current());
+}
+
+void ThemeManager::removeTheme(int index) {
+    if (index < builtInCount()) return;
+    int ui = index - builtInCount();
+    if (ui < 0 || ui >= m_user.size()) return;
+    m_user.remove(ui);
+    if (m_currentIdx == index) {
+        m_currentIdx = 0;
+        emit themeChanged(current());
+    } else if (m_currentIdx > index) {
+        m_currentIdx--;
+    }
+    saveUserThemes();
+}
+
+QString ThemeManager::themesDir() const {
+    QString dir = QStandardPaths::writableLocation(QStandardPaths::AppDataLocation)
+                  + "/themes";
+    QDir().mkpath(dir);
+    return dir;
+}
+
+void ThemeManager::loadUserThemes() {
+    m_user.clear();
+    QDir dir(themesDir());
+    for (const QString& name : dir.entryList({"*.json"}, QDir::Files)) {
+        QFile f(dir.filePath(name));
+        if (!f.open(QIODevice::ReadOnly)) continue;
+        QJsonDocument jdoc = QJsonDocument::fromJson(f.readAll());
+        if (jdoc.isObject())
+            m_user.append(Theme::fromJson(jdoc.object()));
+    }
+}
+
+void ThemeManager::saveUserThemes() const {
+    QString dir = themesDir();
+    // Remove old files
+    QDir d(dir);
+    for (const QString& name : d.entryList({"*.json"}, QDir::Files))
+        d.remove(name);
+    // Write current user themes
+    for (int i = 0; i < m_user.size(); i++) {
+        QString filename = m_user[i].name.toLower().replace(' ', '_') + ".json";
+        QFile f(dir + "/" + filename);
+        if (!f.open(QIODevice::WriteOnly)) continue;
+        f.write(QJsonDocument(m_user[i].toJson()).toJson(QJsonDocument::Indented));
+    }
+}
+
+} // namespace rcx