add FieldSelectionPanel for the ships/stations/debris selection

SelectedBuildingPanel has grown to 1200 lines by carrying two unrelated
selection categories. Introduce the field half as its own widget first,
so the cut-over is a separate, reviewable step.

The panel owns only its own selection state and widgets: it renders the
single-object stats panel (ship, station, debris) or the multi-object
count summary, subscribes to the tick/commands-applied refresh signals
and the debug-draw toggle, and hides itself while it has no selection.
Which category owns the side panel is not its decision - the parent
feeds it through setSelectedEntities/setSelectedDebris/clearSelection.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/ui/CMakeLists.txt

@@ -9,6 +9,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/HeaderBar.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildButtonGrid.h
     ${CMAKE_CURRENT_SOURCE_DIR}/SelectedBuildingPanel.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/FieldSelectionPanel.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintPanel.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutDialog.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutPreview.h
@@ -30,6 +31,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/HeaderBar.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildButtonGrid.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/SelectedBuildingPanel.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/FieldSelectionPanel.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintPanel.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutDialog.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutPreview.cpp