refactor(updates): fetch system-apps updates concurrently

System app update checks were sequential — each app's GitLab API call waited for the previous one to complete. With multiple system apps this added seconds of unnecessary latency on the updates screen.

Run all fetches in parallel using coroutineScope + async so total wait time is bounded by the slowest single request rather than the sum of all requests.

Extract per-app logic into fetchSystemAppUpdate() returning null for skipped apps, and replace early-return chains with null-propagation to make the data flow explicit and avoid lambda-return ambiguity.