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>

tests/test_phase4.cpp

@@ -15320,6 +15320,18 @@ void testTextFitBanner()
         EXPECT_EQ(calls, 6);
     }
 }
+
+// Linux/macOS builds no longer embed res/lang (DRAGONX_EMBED_LANG=OFF), so a translation must load from the
+// loose copy beside the binary (copy_langs puts res/lang in bin/, next to this test). Windows (ON) passes
+// through either the loose or the embedded copy.
+void testLoadLanguageFromLooseFiles()
+{
+    auto& i18n = dragonx::util::I18n::instance();
+    EXPECT_TRUE(i18n.loadLanguage("de"));
+    EXPECT_TRUE(std::string(i18n.translate("overview")) != "Overview");   // de.json: "Übersicht"
+    EXPECT_TRUE(i18n.loadLanguage("en"));                                  // restore the built-in English table
+    EXPECT_EQ(std::string(i18n.translate("overview")), std::string("Overview"));
+}
 }  // namespace
 
 int main()
@@ -15569,6 +15581,7 @@ int main()
     testChatThreadLayout();
     testTextFit();
     testTextFitBanner();
+    testLoadLanguageFromLooseFiles();
 
     if (g_failures != 0) {
         std::cerr << g_failures << " test assertion(s) failed\n";