feat(normalsub): Implement flexible, path-based subscription URLs

Refactors the NormalSub feature to move away from a hardcoded `/sub/normal/` path and a restrictive single-word subpath. This change introduces support for multi-segment, slash-separated subpaths (e.g., `path/to/resource`), providing greater flexibility for creating descriptive and structured subscription URLs.

core/scripts/webpanel/assets/js/settings.js

@@ -88,7 +88,8 @@ $(document).ready(function () {
 
     function isValidSubPath(subpath) {
         if (!subpath) return false;
-        return /^[a-zA-Z0-9]+$/.test(subpath);
+        const subpathRegex = /^[a-zA-Z0-9]+(?:\/[a-zA-Z0-9]+)*$/;
+        return subpathRegex.test(subpath);
     }
 
     function isValidIPorDomain(input) {