Fix Preferences dialog blocking on AI update check

[andriiryzhkov]

Closes #21498

dt_ai_models_check_updates ran synchronously on the GTK main thread when the AI preferences panel opened. Two HTTP calls to GitHub (release listing + versions.json fetch) with a combined 45s timeout worst case – when the network was slow or unreachable, the whole Preferences dialog froze until the calls timed out.

Fix:

• Split the check into a background worker (_check_updates_worker, network + JSON parse only, no registry access) and a main-thread idle callback (_apply_updates_idle, does the registry mutation and raises DT_SIGNAL_AI_MODELS_CHANGED). Preferences opens instantly; the model list auto-refreshes 1–5s later when the worker completes
• Wire the AI preferences panel to listen for DT_SIGNAL_AI_MODELS_CHANGED so it re-populates when the async check finishes
• Tighten curl timeouts on the update-check path from 30s / 15s to 5s + explicit CONNECTTIMEOUT=5s. Caps worst-case background work at ~10s on total network failure

[TurboGit]

Thanks!

[TurboGit]

Needs a release note entry, TIA.