As stated by https://pkg.go.dev/path, "path" is mainly for URLs, "path.filepath" for file systems

- always store resources with a relative path with forward slashes, which will be transformed as needed when the file is accessed

- fix an issue with thumbnail generation on Windows

- add several validations for local storage setting

- improve front-end error feedback when changing local storage

- add migrations to make existing resource paths relative (not needed, but improves database consistency)

fix: Date format in shareable Memo Images (#2668)

Co-authored-by: boojack <stevenlgtm@gmail.com>