Add "You have to create an account to use this feature" lock modal to Saved and Create pages

When user is browsing as guest, the `Saved` and `Create` page is locked for them. According to the Figma design, this needs the gray background modal to (further) show that this feature is locked to them.

Start by locating the Saved and Create page entry points and run the app as a guest to reproduce their locked state. Compare those pages with the referenced Figma design, then verify that the gray lock modal appears on both pages and matches the intended behavior.

Written by the indexing model from the issue text.