build(imgui): compile imgui_demo.cpp to its empty stubs

The ImGui demo window cannot be opened in the wallet (its show flag is
never set), but imgui_demo.cpp was compiled and linked in full.

It is now compiled with IMGUI_DISABLE_DEMO_WINDOWS, which reduces it to
empty stubs. ShowDemoWindow, ShowStyleEditor, ShowAboutWindow,
ShowUserGuide and ShowStyleSelector stay defined, and imgui.h declares
them unconditionally, so a per-file define is ODR-safe.
IMGUI_DISABLE_DEBUG_TOOLS is deliberately not set: it changes imgui.h
declarations and would drop ImGui's duplicate-ID highlight.

Measured:
- imgui_demo.cpp.o: 755,392 -> 1,704 bytes;
- stripped full-node Linux binary: 20,267,680 -> 19,983,136 bytes
  (-284,544);
- full-node Windows release exe: 282,112 bytes smaller;
- the "Dear ImGui Demo" string is gone and the stubs are still defined.

With Makefile generators the per-file define lands in the target's
flags.make, so each existing build tree recompiles the whole app once
(153 files, about 71 s at -j8 here). No runtime or visual change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CMakeLists.txt

@@ -398,6 +398,12 @@ set(IMGUI_SOURCES
     ${IMGUI_DIR}/backends/imgui_impl_sdl3.cpp
 )
 
+# The ImGui demo window is unreachable in the wallet, so compile imgui_demo.cpp to its empty stubs
+# (ShowDemoWindow/ShowStyleEditor/... stay defined; imgui.h declares them unconditionally, so this is ODR-safe).
+# Do NOT use IMGUI_DISABLE_DEBUG_TOOLS: it changes imgui.h declarations and removes duplicate-ID highlighting.
+# (Makefiles: a per-source define lands in the target's flags.make, so existing trees do one full app rebuild.)
+set_source_files_properties(${IMGUI_DIR}/imgui_demo.cpp PROPERTIES COMPILE_DEFINITIONS IMGUI_DISABLE_DEMO_WINDOWS)
+
 set(IMGUI_HEADERS
     ${IMGUI_DIR}/imgui.h
     ${IMGUI_DIR}/imconfig.h