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/desktop_sandbox.h

@@ -0,0 +1,33 @@
+#pragma once
+#include <string>
+#include <memory>
+
+struct lua_State;
+
+namespace mosis {
+
+class TimerManager;
+class VirtualFS;
+
+struct DesktopSandboxConfig {
+    std::string app_id = "com.mosis.designer";
+    std::string data_root = "./sandbox_data";
+};
+
+class DesktopSandbox {
+public:
+    explicit DesktopSandbox(const DesktopSandboxConfig& config);
+    ~DesktopSandbox();
+
+    void RegisterAPIs(lua_State* L);
+    void UnregisterAPIs(lua_State* L);
+    void Update();  // Process timers - call each frame
+    void Reset();   // For hot-reload
+
+private:
+    DesktopSandboxConfig m_config;
+    std::unique_ptr<TimerManager> m_timer_manager;
+    std::unique_ptr<VirtualFS> m_vfs;
+};
+
+} // namespace mosis