Make labels consistent

We shouldn't have different labels for the frontpage and search. They should always be the same and match the UI:

```
_AVAILABILITY_MODES: list[tuple[str, str]] = [
    ("everything",  "Everything"),
    ("ebooks",      "Available to Borrow"),
    ("open_access", "Open Access"),
    ("buyable",     "Available to Purchase"),
]
``` 