Task: Currency pair detail dialog prompts to save with no changes

Opening the Currency Pair detail dialog and immediately closing it (no fields edited) prompts "save changes?" — the dirty-tracking is falsely triggered, most likely by a widget's initial-value-set call path (e.g. a combo box or badge widget's setValue) firing a change signal during dialog population. Find the offending signal/slot wiring and suppress it during initial population.

• [ ] Opening a Currency Pair's detail dialog and closing it immediately (no edits) does not prompt to save.
• [ ] Editing a field and closing still correctly prompts to save (no regression to legitimate dirty-tracking).

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Delivered as part of PR #1472 (task DAE530F1). Root cause: async currency-combo repopulation (setup_currency_combo) fired currentIndexChanged after the dialog had already reset its dirty flag. Fixed with a QSignalBlocker around the repopulation in FlagIconHelper.cpp — a shared fix affecting every entity using flagged_combo fields, not just currency pairs.