[Bug]: backend.log is listed in Settings but cannot be read there #377

What happens

Settings → Logs → Location lists backend.log ("Desktop only: raw output of
the bundled Python backend process") along with its two rotations, and
/api/logs.zip ships them. But the sub-tabs offer only Application log and
Setup log — _LOG_VIEWS has no backend entry.

So the file is advertised in the UI and downloadable, yet unreadable in the app.

Why it matters

backend.log holds what stemdeck.log cannot: anything the process printed
before logging was configured, and anything that killed it before a handler
ran. A backend that dies at startup leaves stemdeck.log empty and the answer
in backend.log — the one log you cannot open.

setup.log is equally desktop-only and does have a tab, so this is an
inconsistency as much as a gap.

Expected

A "Backend log" tab beside the other two, reading backend.log and its
rotations through the same windowed tail endpoint.