feat: surface network errors via a Result wrapper and shared callback

[sinful1992]

Closes #912 (split out from #858).

Problem

Repositories use anonymous Retrofit callbacks whose onFailure is almost always empty (or a bare Log.d), and non-2xx responses in onResponse are ignored. When a request fails the LiveData never updates, so the user sees an empty list or an endless shimmer with no message and no way to tell why — a likely contributor to reports like #853.

Change

This ships the mechanism the issue describes, plus one real consumer, so the rollout to the other repositories can happen incrementally without a 90-file mega-PR.

Mechanism (subsonic/base/):

• Resource<T> — Loading / Success / Error.
• NetworkError — a categorised failure: UNREACHABLE (transport), SERVER_ERROR (non-2xx), INVALID_RESPONSE (2xx but missing/unparseable body), AUTH_FAILED (Subsonic failed with a credential/authorization code), API_ERROR (any other failed). This is the distinction a hardened login needs: wrong credentials vs server unreachable vs server returned garbage.
• SubsonicCallback<T> — one Retrofit adapter that maps all of the above into a Resource in a single place; subclasses only implement extractData(SubsonicResponse). This is what removes the duplicated anonymous-callback boilerplate as callsites migrate.

First consumer — the login credential check:

• SystemRepository.checkUserCredential now reports a Resource<Void>.
• LoginFragment shows a specific message per category (wrong credentials / couldn't reach server / server error / unexpected response) instead of a raw exception string.
• The now-unused SystemCallback interface is removed.

ErrorCode's constants are changed from var to const val so they're usable from Java (they were previously unreferenced).

Deliberately out of scope

The other ~95 enqueue callsites are not touched — they're the incremental follow-up the issue calls for. This PR keeps the diff reviewable and proves the mechanism end-to-end on the path #829 most needs it.

Milestone placement

Per the #858 thread, @eddyizm noted Tom may want to decide where this lands. It's broader than login (it's repository-wide infrastructure) but the login flow is its first consumer, matching the "build it first with LoginActivity as first consumer" plan in #829. Flagging for you both to slot as you see fit.

Testing

Not build-verified locally (no build env set up here) — CI/maintainer build appreciated. The login path is exercised by the existing server-tap flow; error categories map straight from Retrofit/Subsonic responses.

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[sinful1992]

Note: this and #917 (the #911 split) both touch LoginFragment.onServerClick a couple of lines apart, so they're conceptually stacked even though each is branched independently off development for a clean per-PR diff. Whichever merges first, I'll rebase the other — no action needed on your side.