Fix WebDAV saves not visible after relaunch (#338)

[MohandL3G]

This should fix the WebDAV issue from here #338 (comment)

This is what OpenCode wrote:

Root cause: BackupMenuState::initialize_remote_storage compared PROPFIND response parent IDs (full server-relative paths like /remote.php/dav/files/user/JKSV/) against internal m_parent (short config-basepath paths like /JKSV/). The mismatch caused directory_exists to always return false on relaunch, so either a new empty directory was created (MKCOL 201) or initialization returned early (MKCOL 405), making previously uploaded saves invisible or scattering uploads to root.

Fix WebDav::process_listing to use the queried path as parentID instead of extracting it from XML hrefs. Build normalized item IDs from parentID + escaped name, matching the format produced by create_directory()/upload_file(). Also fixes recursive PROPFIND URL construction which previously doubled the path by appending the full href to the origin.