[Models] Mirror custom deck zones in the deck list model

DeckListModel now surfaces the custom zones from the deck tree so
views can render and edit them alongside criteria groups.

The custom-zone bookkeeping that made the model unwieldy is extracted
into DeckListModelCustomZones (deck_list_model_custom_zones.h/.cpp), a
single self-contained unit owning every "what is / where is a custom
zone" decision for the model's shadow tree:

- rebuildTree mirrors each custom zone as a DecklistModelSubZoneNode
  under its board zone, cards flat inside (no further grouping).
- The freshly built shadow tree is sorted while the model reset is
  still open, so views never observe unsorted intermediate order and
  proxies cannot desync.
- Custom zones always sort after criteria groups within a board,
  regardless of their names. One shared sortWithCustomZonesLast backs
  both the live sortHelper (which remaps persistent indexes from the
  movement mapping) and the silent reset-time sortShadowTree.
- addCard inserts flat into a custom zone by name and keeps grouping
  by active criteria for board zones. findCardNode resolves cards in
  both layouts, legacy top-level zones unchanged.
- New IsCustomZoneRole lets views tell zones apart from groups.
- Empty custom zones survive row removal. Zone rows themselves are
  only mutable through the deck tree API.

A new deck_list_model_custom_zones_test suite locks the extracted
shadow-tree logic (type testing, mirroring, name lookup, and the
sort-with-custom-zones-last mapping).

No behavior change.

tests/deck_list_model/CMakeLists.txt

@@ -0,0 +1,33 @@
+add_executable(deck_list_model_custom_zones_test deck_list_model_custom_zones_test.cpp)
+
+if(NOT GTEST_FOUND)
+  add_dependencies(deck_list_model_custom_zones_test gtest)
+endif()
+
+target_link_libraries(
+  deck_list_model_custom_zones_test
+  libcockatrice_models
+  libcockatrice_card
+  libcockatrice_deck_list
+  Threads::Threads
+  ${GTEST_BOTH_LIBRARIES}
+  ${TEST_QT_MODULES}
+)
+add_test(NAME deck_list_model_custom_zones_test COMMAND deck_list_model_custom_zones_test)
+
+add_executable(deck_list_model_zone_integration_test ${VERSION_STRING_CPP} deck_list_model_zone_integration_test.cpp)
+
+if(NOT GTEST_FOUND)
+  add_dependencies(deck_list_model_zone_integration_test gtest)
+endif()
+
+target_link_libraries(
+  deck_list_model_zone_integration_test
+  libcockatrice_models
+  libcockatrice_card
+  libcockatrice_deck_list
+  Threads::Threads
+  ${GTEST_BOTH_LIBRARIES}
+  ${TEST_QT_MODULES}
+)
+add_test(NAME deck_list_model_zone_integration_test COMMAND deck_list_model_zone_integration_test)