feat: add Rated At field - #4653 (#4660)

* feat(model): add Rated At field - #4653

Signed-off-by: zacaj <zacaj@zacaj.com>

* fix(ui): ignore empty dates in rating/love tooltips - #4653

* refactor(ui): add isDateSet util function

Signed-off-by: zacaj <zacaj@zacaj.com>

* feat: add tests for isDateSet and rated_at sort mappings

Added comprehensive tests for isDateSet and urlValidate functions in
ui/src/utils/validations.test.js covering falsy values, Go zero date handling,
valid date strings, Date objects, and edge cases.

Added rated_at sort mapping to album, artist, and mediafile repositories,
following the same pattern as starred_at (sorting by rating first, then by
timestamp). This enables proper sorting by rating date in the UI.

---------

Signed-off-by: zacaj <zacaj@zacaj.com>
Co-authored-by: zacaj <zacaj@zacaj.com>
Co-authored-by: Deluan <deluan@navidrome.org>

persistence/mediafile_repository.go

@@ -84,6 +84,7 @@ func NewMediaFileRepository(ctx context.Context, db dbx.Builder) model.MediaFile
 		"created_at":     "media_file.created_at",
 		"recently_added": mediaFileRecentlyAddedSort(),
 		"starred_at":     "starred, starred_at",
+		"rated_at":       "rating, rated_at",
 	})
 	return r
 }