feat: Online search button

- MailListView: new '🌐 Online' button in search bar
- Clicking button emits onlineSearchRequested(query)
- If search box empty, shows QInputDialog for query entry
- MainMainWindow::onOnlineSearchRequested() handler:
  - Validates current folder selected
  - Gets account for folder
  - Shows status message
  - Shows info dialog with query/folder/account (placeholder for IMAP SEARCH)
- Signal: onlineSearchRequested(const QString& query)
- Button styled in blue (online theme)

build/CMakeFiles/wino-mail-qt.dir/src/ui/maillistview.cpp.o.d

@@ -606,4 +606,8 @@ CMakeFiles/wino-mail-qt.dir/src/ui/maillistview.cpp.o: \
  /usr/include/x86_64-linux-gnu/qt6/QtGui/QDrag \
  /usr/include/x86_64-linux-gnu/qt6/QtGui/qdrag.h \
  /usr/include/x86_64-linux-gnu/qt6/QtCore/QMimeData \
- /usr/include/x86_64-linux-gnu/qt6/QtCore/qmimedata.h
+ /usr/include/x86_64-linux-gnu/qt6/QtCore/qmimedata.h \
+ /usr/include/x86_64-linux-gnu/qt6/QtWidgets/QInputDialog \
+ /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qinputdialog.h \
+ /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qlineedit.h \
+ /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qdialog.h