add sandbox support to desktop designer, fix mouse coordinates and UI issues

- Add DesktopSandbox class that integrates timer, JSON, crypto, and VirtualFS APIs
- Fix mouse coordinate handling: GLFW reports window coordinates, not physical pixels
- Fix font path resolution to search multiple locations for test apps
- Fix screenshot capture timing (capture before buffer swap)
- Fix test app CSS: use border-width instead of border:none, add display:block
- Fix test app Lua: add document nil checks, use HTML entities for symbols
- Update hot_reload to reset sandbox state on reload

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

designer/CMakeLists.txt

@@ -55,6 +55,14 @@ target_compile_definitions(mosis-kernel PUBLIC
     RMLUI_GL3_CUSTOM_LOADER="glad_loader.h"
 )
 
+# Sandbox sources (reuse from MosisService)
+set(SANDBOX_SOURCES
+    ../src/main/cpp/sandbox/timer_manager.cpp
+    ../src/main/cpp/sandbox/json_api.cpp
+    ../src/main/cpp/sandbox/crypto_api.cpp
+    ../src/main/cpp/sandbox/virtual_fs.cpp
+)
+
 # Designer executable
 add_executable(mosis-designer
     main.cpp
@@ -62,6 +70,7 @@ add_executable(mosis-designer
     src/desktop_file_interface.cpp
     src/hot_reload.cpp
     src/platform_singleton.cpp
+    src/desktop_sandbox.cpp
     src/testing/action_recorder.cpp
     src/testing/action_player.cpp
     src/testing/ui_inspector.cpp
@@ -69,6 +78,8 @@ add_executable(mosis-designer
     # Local backend with input recording hooks
     src/backend/RmlUi_Backend_GLFW_GL3.cpp
     src/backend/RmlUi_Platform_GLFW.cpp
+    # Sandbox APIs
+    ${SANDBOX_SOURCES}
 )
 
 target_include_directories(mosis-designer PRIVATE
@@ -77,6 +88,7 @@ target_include_directories(mosis-designer PRIVATE
     src/backend
     ../src/main/kernel/include
     ../src/main/cpp
+    ../src/main/cpp/sandbox
 )
 
 target_link_libraries(mosis-designer PRIVATE