Refactor trending topic aggregation and refine peer check-in detail view

- Decoupled natural participant count from todo item completion stats across trending and topic detail APIs - Refined /trending/[hash] header card to cleanly switch between Today (e.g. 1 person today · 0/1 completed) and All (e.g. 1 person, 7/9 total all-time) tabs - Enhanced peer check-in list to display individual completed progress (e.g. 0/1 or 7/9 completed) with quick status toggle for latest todo instance