Exclude backend/tests from the packaged desktop app

[regnull]

Goal

Stop shipping the backend test suite inside the packaged desktop app.

Context

electron-builder.yml copies ../backend into the app through extraResources with an exclude filter (.venv, caches, databases). backend/tests/ isn't excluded, so every build carries about 530 KB of tests in Resources/backend/tests/ that are never run there.

Acceptance criteria

• backend/tests (and anything else that isn't needed at runtime, if you find more) is added to the extraResources filter
• After make electron-release, the app bundle has no backend/tests/
• The packaged app still starts, its backend comes up, and the UI loads