Add platform abstraction & cross-platform ports

Introduce a cross-platform platform layer and port scaffolding for macOS and Linux. CMakeLists.txt refactored to select platform sources, set executable type per OS, and use CEF helper macros for runtime deployment. Add platform/types.h, startup/paths/browser_host APIs and implementations for Windows, macOS, and Linux (many are stubs for mac/linux). Refactor app entry and lifetime to use nebula::platform::AppStartup (app/main, run.{h,cpp}), move window/browser host logic into platform/browser_host.*, and update NebulaController to use platform APIs (native handles, sizing, visibility, cache-busting token, etc.). Add README and detailed docs/cross-platform.md describing build layout and porting status.

src/platform/mac/browser_host_mac.cpp

@@ -0,0 +1,74 @@
+#include "platform/browser_host.h"
+
+#include <algorithm>
+
+namespace nebula::platform {
+
+CefWindowInfo MakeChildWindowInfo(NativeWindow parent, const Rect& rect) {
+    CefWindowInfo info;
+    info.SetAsChild(parent, CefRect(rect.x, rect.y, rect.width, rect.height));
+    info.runtime_style = CEF_RUNTIME_STYLE_ALLOY;
+    return info;
+}
+
+CefWindowInfo MakeDevToolsPopup(NativeWindow parent, const char* title) {
+    CefWindowInfo info;
+    info.SetAsPopup(parent, title);
+    info.runtime_style = CEF_RUNTIME_STYLE_ALLOY;
+    return info;
+}
+
+void ResizeBrowserWindow(NativeWindow browser_window, const Rect& rect) {
+    UNREFERENCED_PARAMETER(browser_window);
+    UNREFERENCED_PARAMETER(rect);
+}
+
+void SetBrowserVisible(NativeWindow browser_window, bool visible) {
+    UNREFERENCED_PARAMETER(browser_window);
+    UNREFERENCED_PARAMETER(visible);
+}
+
+void RaiseBrowserWindow(NativeWindow browser_window) {
+    UNREFERENCED_PARAMETER(browser_window);
+}
+
+int ScaleForParentWindow(NativeWindow parent, int value) {
+    UNREFERENCED_PARAMETER(parent);
+    return value;
+}
+
+std::pair<int, int> ParentClientSize(NativeWindow parent) {
+    UNREFERENCED_PARAMETER(parent);
+    return {1280, 720};
+}
+
+Rect MenuPopupRect(NativeWindow parent, const BrowserLayout& layout) {
+    const auto [client_right, client_bottom] = ParentClientSize(parent);
+    const int width = 260;
+    const int height = 258;
+    const int margin = 12;
+    const int overlap = 2;
+    const int x = std::max(0, client_right - width - margin);
+    const int y = std::max(0, layout.chrome.y + layout.chrome.height - overlap);
+    return {
+        x,
+        y,
+        std::min(client_right, x + width) - x,
+        std::min(client_bottom, y + height) - y,
+    };
+}
+
+void ApplyRoundedBrowserRegion(NativeWindow browser_window, int corner_radius) {
+    UNREFERENCED_PARAMETER(browser_window);
+    UNREFERENCED_PARAMETER(corner_radius);
+}
+
+std::string CacheBusterToken() {
+    return "0";
+}
+
+void DestroyTopLevelWindow(NativeWindow window) {
+    UNREFERENCED_PARAMETER(window);
+}
+
+}  // namespace nebula::platform