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/src/hot_reload.cpp

@@ -22,14 +22,24 @@ void HotReload::Start() {
 
     m_running = true;
 #ifdef _WIN32
+    // Create a manual-reset event for signaling shutdown
+    m_stop_event = CreateEventW(nullptr, TRUE, FALSE, nullptr);
+    if (!m_stop_event) {
+        std::cerr << "Failed to create stop event" << std::endl;
+        m_running = false;
+        return;
+    }
+
     m_notification_handle = FindFirstChangeNotificationW(
         m_watch_path.c_str(),
         TRUE,  // Watch subtree
         FILE_NOTIFY_CHANGE_LAST_WRITE
     );
-    
+
     if (m_notification_handle == INVALID_HANDLE_VALUE) {
         std::cerr << "Failed to set up file watching" << std::endl;
+        CloseHandle(m_stop_event);
+        m_stop_event = nullptr;
         m_running = false;
         return;
     }
@@ -39,6 +49,12 @@ void HotReload::Start() {
 
 void HotReload::Stop() {
     m_running = false;
+#ifdef _WIN32
+    // Signal the stop event to wake up the watch thread
+    if (m_stop_event) {
+        SetEvent(m_stop_event);
+    }
+#endif
     if (m_watch_thread.joinable()) {
         m_watch_thread.join();
     }
@@ -47,21 +63,33 @@ void HotReload::Stop() {
         FindCloseChangeNotification(m_notification_handle);
         m_notification_handle = nullptr;
     }
+    if (m_stop_event) {
+        CloseHandle(m_stop_event);
+        m_stop_event = nullptr;
+    }
 #endif
 }
 
 void HotReload::WatchThread() {
     while (m_running) {
 #ifdef _WIN32
-        DWORD result = WaitForSingleObject(m_notification_handle, 100);  // 100ms timeout
+        // Wait on both the file change notification and the stop event
+        HANDLE handles[2] = { m_notification_handle, m_stop_event };
+        DWORD result = WaitForMultipleObjects(2, handles, FALSE, INFINITE);
+
         if (result == WAIT_OBJECT_0) {
+            // File change notification
             // Debounce - wait a bit for file writes to complete
             std::this_thread::sleep_for(std::chrono::milliseconds(100));
-            if (m_callback) {
+            if (m_callback && m_running) {
                 m_callback();
             }
             FindNextChangeNotification(m_notification_handle);
+        } else if (result == WAIT_OBJECT_0 + 1) {
+            // Stop event signaled - exit the loop
+            break;
         }
+        // WAIT_FAILED or other errors will just loop and check m_running
 #else
         // TODO: Linux inotify / macOS FSEvents implementation
         std::this_thread::sleep_for(std::chrono::milliseconds(100));