Make trigger config dialog backend-aware

Introduce a TriggerUiProfile dataclass and trigger_ui_profile_for_backend() to drive dialog presentation per backend. Replace free-text source field with an editable QComboBox providing backend suggestions and defaults. Add profile-driven visibility/enabling for input, master, software and strobe/line output fields, plus helper methods to manage form rows and combo text. Only include strobe-related payload fields when the backend profile exposes them. Misc: expand info/help text, show backend in group title, increase dialog min-width, refine tooltips, and improve model↔UI mapping and payload construction.

Author: C-Achard