Terminology updates and unread count cap

The sidebar and inbox page now say "threads" instead of "discussions" for unread and direct items, aligning terminology across the app. The search modal's content-type dropdown now offers "Polls" instead of the older "Decisions" label.

The inbox page and sidebar unread counter now only consider activity from the last 6 weeks, with an explanatory notice added to the bottom of the inbox page. This keeps long-idle threads from cluttering unread counts indefinitely.