Fix SFTP root navigation on Windows

Use portable POSIX parent paths instead of platform-dependent dirname() for remote navigation, previews, uploads, file operations and shared URLs.

Normalize browser paths, avoid doubled root separators and preserve top-level directory names in ZIP downloads.

Add regression coverage for root navigation, generated links, relative paths and ZIP members, and document the new test command. Verified with PHP syntax checks and all three regression suites.

Web/content/ftp/upload.php

@@ -20,7 +20,7 @@ function createDirectoryRecursive($sftp, $path) {
         return true;
     }
     
-    $parent = dirname($path);
+    $parent = remoteDirname($path);
     if ($parent != '/' && !$sftp->is_dir($parent)) {
         createDirectoryRecursive($sftp, $parent);
     }
@@ -91,7 +91,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
         if (!empty($relativePath)) {
             $fileName = basename($relativePath);
 
-            $dirPart = dirname($relativePath);
+            $dirPart = remoteDirname($relativePath);
             if ($dirPart !== '.' && $dirPart !== '') {
                 $remoteDirPath = $currentPath . '/' . $dirPart;