feat: responsive compact mail view + folder unread badges

- CompactMailDelegate: card-based view for narrow panels (<420px)
  * Avatar with hash-based color, sender + date (relative format)
  * Subject line with attachment icon
  * Action buttons: flag, delete, category, more (context menu)
  * Unread highlight with light blue background + bold sender
- MailListView: automatic switch between tree/table (wide) and list/cards (narrow)
  * QStackedWidget with QTreeView + QListView sharing same proxy model
  * resizeEvent threshold at 420px
  * Signals forwarded to MainMainWindow for actions
- FolderTreeDelegate: custom paint for folder tree
  * Unread count badge (blue pill) right-aligned on folder nodes
  * Account nodes bold, folder nodes normal weight
  * Proper indentation per depth level
- MailItemDao::countUnreadByFolderId() for real-time unread counts
- MainMainWindow handlers for compact actions (flag, delete, category, more menu)

thirdparty/boost/boost_headers-1.90.0/boost_headers-config.cmake

@@ -0,0 +1,35 @@
+# Generated by Boost 1.90.0
+
+if(TARGET Boost::headers)
+  return()
+endif()
+
+if(Boost_VERBOSE OR Boost_DEBUG)
+  message(STATUS "Found boost_headers ${boost_headers_VERSION} at ${boost_headers_DIR}")
+endif()
+
+mark_as_advanced(boost_headers_DIR)
+
+# Compute the include and library directories relative to this file.
+
+get_filename_component(_BOOST_CMAKEDIR "${CMAKE_CURRENT_LIST_DIR}/../" REALPATH)
+
+# If the computed and the original directories are symlink-equivalent, use original
+if(EXISTS "/build/boost1.90-Jxx13U/boost1.90-1.90.0/debian/tmp/usr/lib/x86_64-linux-gnu/cmake")
+  get_filename_component(_BOOST_CMAKEDIR_ORIGINAL "/build/boost1.90-Jxx13U/boost1.90-1.90.0/debian/tmp/usr/lib/x86_64-linux-gnu/cmake" REALPATH)
+  if(_BOOST_CMAKEDIR STREQUAL _BOOST_CMAKEDIR_ORIGINAL)
+    set(_BOOST_CMAKEDIR "/build/boost1.90-Jxx13U/boost1.90-1.90.0/debian/tmp/usr/lib/x86_64-linux-gnu/cmake")
+  endif()
+  unset(_BOOST_CMAKEDIR_ORIGINAL)
+endif()
+
+get_filename_component(_BOOST_INCLUDEDIR "${_BOOST_CMAKEDIR}/../../../include/" ABSOLUTE)
+
+add_library(Boost::headers INTERFACE IMPORTED)
+
+set_target_properties(Boost::headers PROPERTIES
+  INTERFACE_INCLUDE_DIRECTORIES "${_BOOST_INCLUDEDIR}"
+)
+
+unset(_BOOST_INCLUDEDIR)
+unset(_BOOST_CMAKEDIR)