Fix desktop designer click handling and add goHome API

Designer click handling:
- Fix DPI scaling in MouseButtonCallback and CursorPosCallback
- Scale coordinates from window space to framebuffer/RmlUi context
- Remove window resizing in ResizeToPhone (caused DPI mismatches)

Test framework:
- Fix SendMouseDown to use MOUSEEVENTF_MOVE before button down
- Remove double-scaling in ScaleToPhysical (WindowController handles it)
- All 5 UI navigation tests now pass

Kernel API:
- Add goHome() Lua function to return to home screen
- Stops any running third-party apps before navigating

Test app:
- Update sandbox-test to use goHome() instead of goBack()

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

src/main/cpp/kernel.cpp

@@ -257,13 +257,65 @@ static int LuaLoadScreen(lua_State* L)
     return 1;
 }
 
+// Lua function to go back to home screen
+static int LuaGoHome(lua_State* L)
+{
+    if (!g_context)
+    {
+        lua_pushboolean(L, false);
+        return 1;
+    }
+
+    Logger::Log("goHome called - returning to home screen");
+
+    // Stop any running third-party app
+    if (g_sandbox_manager) {
+        auto running_apps = g_sandbox_manager->GetRunningApps();
+        for (const auto& app_id : running_apps) {
+            // Don't stop system apps
+            if (app_id.find("com.mosis.") == 0 && app_id != "com.mosis.home") {
+                Logger::Log(std::format("Stopping app: {}", app_id));
+                g_sandbox_manager->StopApp(app_id);
+            }
+        }
+    }
+
+    // Load home screen
+    const char* home_path = "apps/home/home.rml";
+
+    // Unload current document
+    if (g_document)
+    {
+        g_context->UnloadDocument(g_document);
+        g_document = nullptr;
+    }
+
+    // Load home document
+    g_document = g_context->LoadDocument(home_path);
+    if (g_document)
+    {
+        g_document->Show();
+        Logger::Log("Returned to home screen");
+        lua_pushboolean(L, true);
+    }
+    else
+    {
+        Logger::Log("Failed to load home screen");
+        lua_pushboolean(L, false);
+    }
+
+    return 1;
+}
+
 // Register Lua functions for navigation
 static void RegisterLuaFunctions(const std::string& current_app_id, bool is_system_app)
 {
     lua_State* L = Rml::Lua::Interpreter::GetLuaState();
     lua_pushcfunction(L, LuaLoadScreen);
     lua_setglobal(L, "loadScreen");
-    Logger::Log("Registered Lua loadScreen function");
+    lua_pushcfunction(L, LuaGoHome);
+    lua_setglobal(L, "goHome");
+    Logger::Log("Registered Lua loadScreen and goHome functions");
 
     // Register app management APIs
     if (g_app_manager && g_update_service) {