Fix delta updates deleting nested files on Windows (#486)

GameDownloadAgent::run() prunes the install directory by comparing
locally-scanned relative paths against the manifest's file_list keys.
file_list is always written with `/` separators (see droplet's
path_backend.rs), regardless of which OS produced the manifest, but
Path::strip_prefix().to_string_lossy() on Windows yields `\`-separated
strings. Every nested file therefore fails the exact-string HashMap
lookup and gets deleted on every download/update, while flat root
files (no separator to disagree on) happen to survive. This is most
visible on Unity games, where nearly all content lives under a deeply
nested *_Data folder.

Confirmed against a live Drop server: 401/407 of a real game's
file_list entries contain `/`, with the remaining 6 being
separator-less root files, matching exactly which files survive vs.
get pruned.

desktop/src-tauri/games/src/downloads/download_agent.rs

@@ -308,7 +308,7 @@ impl GameDownloadAgent {
         let current_file_tree = self.scan_filetree(base_path)?;
 
         for file in current_file_tree {
-            let filename = file.strip_prefix(base_path)?.to_string_lossy().to_string();
+            let filename = file.strip_prefix(base_path)?.to_string_lossy().replace('\\', "/");
             let needed = file_list.contains_key(&filename) || filename == ".dropdata";
             if !needed {
                 debug!("deleted {}", file.display());