Add Directory Monitoring handling to File-Not-Found page

Directory Monitoring can remove zims from the library instance. We now only remove zims that are not opened by the user. There could be the case where library instance is directly modified inside kiwix-desktop, filesystem, etc that causes us to lose the zim information. We only display N/A in this case for path and name on the page.

src/contentmanager.cpp

@@ -906,5 +906,6 @@ void ContentManager::updateLibraryFromDir(QString monitorDir)
 
 void ContentManager::handleDisappearedZimFile(QString bookId)
 {
-    mp_library->removeBookFromLibraryById(bookId);
+    if (!KiwixApp::instance()->getTabWidget()->getTabZimIds().contains(bookId))
+        mp_library->removeBookFromLibraryById(bookId);
 }

src/kiwixapp.cpp

@@ -106,10 +106,10 @@ void KiwixApp::init()
         mp_manager->asyncUpdateLibraryFromDir(monitorDir);
     });
 
-    setupDirectoryMonitoring();
-
+    /* Restore Tabs before directory monitoring to ensure we know what tabs user had. */
     restoreTabs();
     restoreWindowState();
+    setupDirectoryMonitoring();
 }
 
 void KiwixApp::setupDirectoryMonitoring()

src/urlschemehandler.cpp

@@ -181,9 +181,15 @@ UrlSchemeHandler::replyZimNotFoundPage(QWebEngineUrlRequestJob *request,
                                        const QString &zimId)
 {
     QBuffer *buffer = new QBuffer;
-    auto& book = KiwixApp::instance()->getLibrary()->getBookById(zimId);
-    QString path = QString::fromStdString(book.getPath());
-    QString name = QString::fromStdString(book.getName());
+    QString path = "N/A", name = "N/A";
+    try 
+    {
+        auto& book = KiwixApp::instance()->getLibrary()->getBookById(zimId);
+        path = QString::fromStdString(book.getPath());
+        name = QString::fromStdString(book.getName());
+    }
+    catch (...) { /* Blank */ }
+
     QString contentHtml = "<section><div>"
                           "<h1>" +
                           gt("file-not-found-title") +