Fix icon loading for third-party apps with file:// URL scheme

- Add file:// URL handling to AssetFilesInterface in kernel.cpp
- Update home.lua to use file:// prefix for absolute filesystem paths
- Add file:// URL handling to desktop file interface for consistency

This fixes RmlUi stripping the leading slash from absolute paths
when resolving img src URLs relative to the document base.

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

designer/src/desktop_file_interface.cpp

@@ -19,6 +19,11 @@ void DesktopFileInterface::SetAssetsPath(const std::string& path) {
 std::string DesktopFileInterface::ResolvePath(const std::string& path) const {
     std::string resolved = path;
 
+    // Handle file:// URLs
+    if (resolved.rfind("file://", 0) == 0) {
+        resolved = resolved.substr(7);  // Strip "file://"
+    }
+
     // Handle URL-encoded Windows drive letters (D| -> D:)
     // RmlUi sometimes encodes the colon in Windows paths
     if (resolved.size() >= 2 && std::isalpha(resolved[0]) && resolved[1] == '|') {