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>

src/util/i18n.cpp

@@ -9,9 +9,14 @@
 #include <fstream>
 #include <cstdio>
 #include <cstring>
+#include <vector>
 #include <nlohmann/json.hpp>
 
-// Embedded language files
+#ifndef DRAGONX_EMBED_LANG
+#define DRAGONX_EMBED_LANG 1   // legacy build paths keep the old behaviour
+#endif
+#if DRAGONX_EMBED_LANG
+// Embedded language files (Windows single-file exe has no res/lang)
 #include "embedded/lang_es.h"
 #include "embedded/lang_zh.h"
 #include "embedded/lang_ru.h"
@@ -20,6 +25,7 @@
 #include "embedded/lang_pt.h"
 #include "embedded/lang_ja.h"
 #include "embedded/lang_ko.h"
+#endif
 #include "../util/logger.h"
 
 namespace dragonx {
@@ -109,10 +115,14 @@ bool I18n::loadLanguage(const std::string& locale)
 {
     // Try to load from file: CWD-relative first, then exe-relative
     std::string rel_path = "res/lang/" + locale + ".json";
-    std::string exe_path = Platform::getExecutableDirectory() + "/" + rel_path;
-
-    // Try both paths; prefer whichever opens successfully
-    for (const auto& lang_file : { rel_path, exe_path }) {
+    const std::string exe_dir = Platform::getExecutableDirectory();
+    std::vector<std::string> candidates = { rel_path, exe_dir + "/" + rel_path };
+#ifndef _WIN32
+    // Layouts where res/ is not beside the binary, independent of the launcher's cd:
+    candidates.push_back(exe_dir + "/../share/" DRAGONX_APP_NAME "/" + rel_path);  // cmake --install; full AppImage
+    candidates.push_back(exe_dir + "/../Resources/" + rel_path);                     // macOS .app (.bin run directly)
+#endif
+    for (const auto& lang_file : candidates) {
         std::ifstream file(lang_file);
         if (!file.is_open()) continue;
 
@@ -137,6 +147,7 @@ bool I18n::loadLanguage(const std::string& locale)
         }
     }
     
+#if DRAGONX_EMBED_LANG
     // Try embedded language data
     const unsigned char* embedded_data = nullptr;
     unsigned int embedded_size = 0;
@@ -188,6 +199,7 @@ bool I18n::loadLanguage(const std::string& locale)
             DEBUG_LOGF("Error parsing embedded language %s: %s\n", locale.c_str(), e.what());
         }
     }
+#endif
     
     // If English, use built-in strings
     if (locale == "en") {