ci: add emulator tests against dockerized Subsonic servers

[xsteadfastx]

What this does

Adds a CI workflow that actually boots an Android emulator and runs a real instrumented test against a real Subsonic server. No more "it works on my machine" — the app now proves it can install, launch, connect to a server, and render the home screen, on every push.

How it works

The workflow spins up a dockerized Subsonic server, seeds a tiny test library (two sine-wave MP3s generated with ffmpeg), boots an emulator with KVM, and runs a smoke test that:

1. launches the app
2. waits for it to reach a usable state
3. checks it actually connects to the server (not just that it opens)
4. scrolls to the Discovery section and asserts a seeded song shows up

Coverage

It runs the same test across a small matrix:

Server	Flavor
Navidrome	tempus
Navidrome	degoogled
gonic	tempus
gonic	degoogled

So we get both server implementations and both product flavors covered for free.

Why this matters

The existing unit tests are great, but they never touch a real server or a real device. This catches the kind of thing that only shows up when the app talks to an actual Subsonic API — wrong endpoints, auth issues, library parsing, that sort of thing. And since it runs on every push, regressions get caught early instead of at release time.

Notes

• The smoke test is intentionally small. It proves the plumbing works (emulator + server + app + library), not every feature. Playback, downloads, bookmarks, etc. can be added as follow-ups.
• The server password is passed to the test via an instrumentation argument, so each server in the matrix logs in with its own credentials.
• gonic needed a couple of non-obvious settings to start in docker (it listens on port 80 by default and won't boot without a playlists dir) — that's all handled in the workflow.

Testing

All four matrix jobs pass on the ci/emulator-tests branch.

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