Add cross-platform path handling (#89)

Add cross-platform path handling support

- Add sanitizePath, joinPath, buildOutputPath utilities
- Add operatingSystem to Settings interface
- Replace hardcoded Windows paths with dynamic path handling
- Support Windows, Linux, and macOS

frontend/src/lib/utils.ts

@@ -1,6 +1,35 @@
 import { clsx, type ClassValue } from "clsx"
 import { twMerge } from "tailwind-merge"
+import type { Settings } from "./settings";
 
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs))
 }
+
+
+export function sanitizePath(input: string, os: string): string {
+  if (os === "Windows") {
+    return input.replace(/[<>:"/\\|?*]/g, "_");
+  }
+
+  // unix-based OS
+  return input.replace(/\//g, "_");
+}
+
+export function joinPath(os: string, ...parts: string[]): string {
+  const sep = os === "Windows" ? "\\" : "/";
+
+  return parts
+    .filter(Boolean)
+    .map(p => p.replace(/^[/\\]+|[/\\]+$/g, ""))
+    .join(sep);
+}
+
+export function buildOutputPath(settings: Settings, folder?: string) {
+  const os = settings.operatingSystem;
+
+  const base = settings.downloadPath || "";
+  const sanitized = folder ? sanitizePath(folder, os) : undefined;
+
+  return sanitized ? joinPath(os, base, sanitized) : base;
+}