feat: move-to-folder dialog, batch actions, reader source view, Focused/Other pivot

- MoveToFolderDialog: modal folder picker per account (excludes current)
- Wire 'Mover a...' in reader ⋯ menu + batch move (emit -1 -> dialog)
- Connect batch delete/mark-unread/flag handlers (were unconnected)
- Reader: 'Ver codigo fuente' reads .eml via MimeStorageService::readEmlFile
- Reader: 'Marcar como no leido' emits markUnreadRequested
- Pivot Focused/Other: EmailListModel::setShowFocusedOnly heuristic
  (flagged/pinned/frequent senders), reset to Focused on inbox select

src/ui/movetofolderdialog.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include <QDialog>
+#include <QListWidget>
+#include "core/models/folder.h"
+
+/// Modal dialog to pick a target folder for moving one or more mails.
+class MoveToFolderDialog : public QDialog
+{
+    Q_OBJECT
+public:
+    explicit MoveToFolderDialog(const QVector<Folder> &folders, int excludeFolderId = -1,
+                                QWidget *parent = nullptr);
+
+    /// Target folder id, or -1 if cancelled.
+    int selectedFolderId() const { return m_selectedId; }
+
+private slots:
+    void accept() override;
+
+private:
+    QListWidget *m_list;
+    int m_selectedId = -1;
+};