build(i18n): embed language JSON only on Windows, add lookup fallbacks

CMake turned every res/lang/*.json into an xxd header that i18n.cpp
compiled in as a fallback, on every platform. Only the Windows
single-file exe needs it: the Linux zip and AppImage and the macOS .app
always ship res/lang, and loadLanguage() reads the loose file first. On
Linux the embedded copy added about 1.5 MB to the binary, and every
translation edit recompiled i18n.cpp.

A new option, DRAGONX_EMBED_LANG, defaults to ON for WIN32 and OFF
elsewhere:
- ON without xxd is now a configure error (it used to warn, then fail to
  compile i18n.cpp on the missing headers);
- i18n.cpp compiles the embedded includes and the fallback only when ON;
- outside Windows, loadLanguage() also tries
  <exe>/../share/<app>/res/lang (the cmake --install layout and the
  full-node AppImage) and <exe>/../Resources/res/lang (the macOS .app
  binary run directly), so those layouts find the loose files whatever
  the working directory; before, only the embedded copy covered that.

Measured:
- full-node Linux binary: .data 1,699,696 -> 168,592 bytes; stripped
  19,983,136 -> 18,456,128 bytes (-1.53 MB);
- with the ImGui demo stubs, the Linux zips and AppImages are
  0.50-0.55 MB smaller, with identical file lists including all 8
  language files;
- a Lite build started from an unrelated working directory loads the
  ja.json next to the binary (2422 strings), and the packaged Lite zip
  and AppImage load ja.json with no new not-found or failed lines;
- Windows keeps the embedded copy: the 8 language headers are generated
  and the ja.json bytes are in the exe.

A new test loads "de" from the loose files and checks a translated
string; with bin/res/lang moved away exactly its two assertions fail.

Behaviour changes on Linux and macOS: a missing or corrupt res/lang now
falls back to English instead of the embedded copy, and a translation
edit needs only the copy_langs step instead of a recompile.

Not verifiable here: the macOS .app in a non-English language, and the
Windows single-file exe at runtime.

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

CMakeLists.txt

@@ -46,6 +46,14 @@ option(DRAGONX_BUILD_LITE "Build ObsidianDragonLite variant without full-node fe
 option(DRAGONX_ENABLE_LITE_BACKEND "Enable real lite wallet backend integration" OFF)
 option(DRAGONX_ENABLE_CHAT "Enable the HushChat protocol/UI integration" ON)
 option(DRAGONX_BUILD_FAKE_DAEMON "Build the fake slow dragonxd test stub (tools/fake_dragonxd); never shipped" OFF)
+# Embedded fallback copy of res/lang/*.json (xxd -i -> lang_*.h). Needed only by the Windows single-file exe (no res/ beside it);
+# Linux zip/AppImage and the macOS .app always ship res/lang, which loadLanguage() reads first.
+if(WIN32)
+    set(_DRAGONX_EMBED_LANG_DEFAULT ON)
+else()
+    set(_DRAGONX_EMBED_LANG_DEFAULT OFF)
+endif()
+option(DRAGONX_EMBED_LANG "Embed res/lang/*.json in the binary as a fallback" ${_DRAGONX_EMBED_LANG_DEFAULT})
 set(DRAGONX_LITE_BACKEND_LIBRARY "" CACHE FILEPATH "Path to a prebuilt SDXL-compatible lite backend library")
 set(DRAGONX_LITE_BACKEND_INCLUDE_DIR "" CACHE PATH "Optional include directory for SDXL-compatible lite backend headers")
 set(DRAGONX_LITE_BACKEND_EXTRA_LIBS "" CACHE STRING "Additional libraries needed by the SDXL-compatible lite backend")
@@ -929,6 +937,7 @@ target_compile_definitions(ObsidianDragon PRIVATE
     DRAGONX_ENABLE_EMBEDDED_DAEMON=$<BOOL:${DRAGONX_ENABLE_EMBEDDED_DAEMON}>
     DRAGONX_ENABLE_LITE_BACKEND=$<BOOL:${DRAGONX_LITE_BACKEND_READY}>
     DRAGONX_ENABLE_CHAT=$<BOOL:${DRAGONX_ENABLE_CHAT}>
+    DRAGONX_EMBED_LANG=$<BOOL:${DRAGONX_EMBED_LANG}>
 )
 if(WIN32)
     target_compile_definitions(ObsidianDragon PRIVATE DRAGONX_USE_DX11)
@@ -990,9 +999,11 @@ endif()
 # so edits to res/lang/*.json are picked up by 'make' without re-running cmake.
 file(GLOB LANG_FILES ${CMAKE_SOURCE_DIR}/res/lang/*.json)
 if(LANG_FILES)
-    find_program(XXD_EXECUTABLE NAMES xxd)
-    if(NOT XXD_EXECUTABLE)
-        message(WARNING "xxd not found; runtime language JSON files will be copied, but embedded build/generated/embedded/lang_*.h files will not be regenerated")
+    if(DRAGONX_EMBED_LANG)
+        find_program(XXD_EXECUTABLE NAMES xxd)
+    endif()
+    if(DRAGONX_EMBED_LANG AND NOT XXD_EXECUTABLE)
+        message(FATAL_ERROR "DRAGONX_EMBED_LANG is ON but xxd was not found; it generates build/generated/embedded/lang_*.h. Install xxd or configure with -DDRAGONX_EMBED_LANG=OFF")
     endif()
     file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/generated/embedded)
 
@@ -1009,7 +1020,7 @@ if(LANG_FILES)
         list(APPEND LANG_OUTPUTS ${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/res/lang/${LANG_FILENAME})
 
         # Also regenerate the embedded header so the binary always has fresh translations
-        if(XXD_EXECUTABLE)
+        if(DRAGONX_EMBED_LANG AND XXD_EXECUTABLE)
             get_filename_component(LANG_CODE ${LANG_FILENAME} NAME_WE)
             set(LANG_HEADER ${CMAKE_BINARY_DIR}/generated/embedded/lang_${LANG_CODE}.h)
             add_custom_command(
@@ -1286,6 +1297,7 @@ if(BUILD_TESTING)
         DRAGONX_LITE_BUILD=$<BOOL:${DRAGONX_BUILD_LITE}>
         DRAGONX_ENABLE_EMBEDDED_DAEMON=$<BOOL:${DRAGONX_ENABLE_EMBEDDED_DAEMON}>
         DRAGONX_ENABLE_LITE_BACKEND=$<BOOL:${DRAGONX_LITE_BACKEND_READY}>
+        DRAGONX_EMBED_LANG=$<BOOL:${DRAGONX_EMBED_LANG}>
         DRAGONX_TEST_FIXTURE_DIR="${CMAKE_SOURCE_DIR}/tests/fixtures"
     )