Add host platform detection and macOS styles

Expose host platform to the UI and apply platform-specific styling. Backend: add CurrentPlatformName() and include "platform" in the JSON sent from NebulaController so the frontend can know the host OS. Frontend: detectHostPlatform() initializes state.platform, applyPlatform() sets a platform-* body class, and applyState() applies it. CSS: add .platform-macos rules to adjust title padding and hide window controls on macOS. Also fix FilePathToUrl to avoid producing an extra slash when the encoded path already starts with '/'.

src/ui/paths.cpp

@@ -174,7 +174,7 @@ std::string FilePathToUrl(std::filesystem::path path) {
     for (char ch : value) {
         encoded += ch == ' ' ? "%20" : std::string(1, ch);
     }
-    return "file:///" + encoded;
+    return encoded.starts_with('/') ? "file://" + encoded : "file:///" + encoded;
 }
 
 std::string GetChromeUrl() {