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

CMakeLists.txt

@@ -108,6 +108,7 @@ set(SRC_FILES
     src/core/synchronizerprovider.cpp
     src/core/accountsetupdialoglauncher.cpp
     src/core/oauthcallbackserver.cpp
+    src/core/icloudauthenticator.cpp
     src/ui/mainmainwindow.cpp
     src/ui/mainmainwindow.h
     src/ui/newmessagedialog.cpp
@@ -138,6 +139,8 @@ set(SRC_FILES
     src/ui/categorytreewidget.h
     src/ui/rulesmanagerdialog.cpp
     src/ui/rulesmanagerdialog.h
+    src/ui/movetofolderdialog.cpp
+    src/ui/movetofolderdialog.h
     src/ui/ruleditordialog.cpp
     src/ui/ruleditordialog.h
     src/ui/templateeditordialog.cpp