fix(gui): expand tildes for subdir check (fixes #9400) (#9788)

### Purpose

This closes #9400 by always expanding tildes when parent/subdir checks
are done.

### Testing

I tested this by creating folders with paths to parent or subdirectories
of the default folder that include a tilde in their path as shown in the
attached screenshots.
With this change, overlap will be detected regardless of wether or not
tildes are used in other folder paths.

### Screenshots

Default Folder:

![2024-10-26-At-08h40m33s](https://github.com/user-attachments/assets/07df090c-4481-41ec-b741-d2785fc848d5)
Newly created folder (parent directory in this case)

![2024-10-26-At-08h40m13s](https://github.com/user-attachments/assets/636fa1fd-41dc-44d9-ac90-0a4937c9921c)

---------

Signed-off-by: tobifroe <[email protected]>

gui/default/syncthing/core/pathIsSubDirDirective.js

@@ -6,7 +6,12 @@ angular.module('syncthing.core')
                 ctrl.$validators.folderPathErrors = function (viewValue) {
                     // This function checks whether ydir is a subdirectory of xdir,
                     // e.g. it would return true if xdir = "/home/a", ydir = "/home/a/b".
+                    // Tildes in both xdir and ydir are expanded for comparison
+                    // so that e.g. xdir = "home/a/", ydir = "~/b" will return true.
                     function isSubDir(xdir, ydir) {
+                        var tildeExpansionRegex = new RegExp(`^~${scope.system.pathSeparator}|^~/`);
+                        xdir = xdir.replace(tildeExpansionRegex, `${scope.system.tilde}${scope.system.pathSeparator}`);
+                        ydir = ydir.replace(tildeExpansionRegex, `${scope.system.tilde}${scope.system.pathSeparator}`);
                         var xdirArr = xdir.split(scope.system.pathSeparator);
                         var ydirArr = ydir.split(scope.system.pathSeparator);
                         if (xdirArr.slice(-1).pop() === "") {