fix: normalize Windows backslash paths in weed admin file uploads (#7636)

fix: normalize Windows backslash paths in file uploads

When uploading files from a Windows client to a Linux server,
file paths containing backslashes were not being properly interpreted as
directory separators. This caused files intended for subdirectories to be
created in the root directory with backslashes in their filenames.

Changes:
- Add util.CleanWindowsPath and util.CleanWindowsPathBase helper functions
  in weed/util/fullpath.go for reusable path normalization
- Use path.Join/path.Clean/path.Base instead of filepath equivalents
  for URL path semantics (filepath is OS-specific)
- Apply normalization in weed admin handlers and filer upload parsing

Fixes #7628

weed/util/fullpath.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"path"
 	"path/filepath"
 	"strings"
 )
@@ -85,3 +86,15 @@ func StringSplit(separatedValues string, sep string) []string {
 	}
 	return strings.Split(separatedValues, sep)
 }
+
+// CleanWindowsPath normalizes Windows-style backslashes to forward slashes.
+// This handles paths from Windows clients where paths use backslashes.
+func CleanWindowsPath(p string) string {
+	return strings.ReplaceAll(p, "\\", "/")
+}
+
+// CleanWindowsPathBase normalizes Windows-style backslashes to forward slashes
+// and returns the base name of the path.
+func CleanWindowsPathBase(p string) string {
+	return path.Base(strings.ReplaceAll(p, "\\", "/"))
+}