add QCanvas Root and viewport pane in gcompose

app/gcompose/src/MainPanel.cpp

@@ -0,0 +1,49 @@
+#include "MainPanel.h"
+#include "QViewportPane.h"
+#include <QVBoxLayout>
+#include <QHBoxLayout>
+#include <QSplitter>
+#include <QLabel>
+#include <QPushButton>
+
+MainPanel::MainPanel(QWidget* parent) : QWidget(parent) {
+    auto* mainLayout = new QVBoxLayout(this);
+    mainLayout->setContentsMargins(0, 0, 0, 0);
+    mainLayout->setSpacing(0);
+
+    // 1. Top Menu Panel
+    auto* menuPanel = new QWidget(this);
+    menuPanel->setFixedHeight(36);
+    menuPanel->setStyleSheet("background-color: #2b2b2b; border-bottom: 1px solid #111;");
+    
+    auto* menuLayout = new QHBoxLayout(menuPanel);
+    menuLayout->setContentsMargins(10, 0, 10, 0);
+    menuLayout->setSpacing(15);
+
+    auto* logo = new QLabel("G-COMPOSE", menuPanel);
+    logo->setStyleSheet("font-weight: bold; color: #0078d7; font-size: 14px; letter-spacing: 1px;");
+
+    auto* btnOpen = new QPushButton("Open", menuPanel);
+    auto* btnSave = new QPushButton("Save", menuPanel);
+    
+    QString btnStyle = "QPushButton { background: transparent; color: #ccc; border: none; padding: 5px 10px; }"
+                       "QPushButton:hover { background: #3c3c3c; color: white; border-radius: 4px; }";
+    btnOpen->setStyleSheet(btnStyle);
+    btnSave->setStyleSheet(btnStyle);
+
+    menuLayout->addWidget(logo);
+    menuLayout->addWidget(btnOpen);
+    menuLayout->addWidget(btnSave);
+    menuLayout->addStretch();
+
+    mainLayout->addWidget(menuPanel);
+
+    // 2. Central Splitter Area
+    m_rootSplitter = new QSplitter(Qt::Horizontal, this);
+    m_firstPane = new QViewportPane(m_rootSplitter);
+    m_rootSplitter->addWidget(m_firstPane);
+
+    mainLayout->addWidget(m_rootSplitter, 1);
+}
+
+MainPanel::~MainPanel() {}