fix: compute mock activity timestamps at call time, not module load

[smartcalsystems-sketch]

Problem

recentActivity used minutesAgo(n), computed once when mock-data.ts is first imported. On a long-lived Next.js server, the module stays loaded, so a day after deploy the homepage's "Live on the platform" feed reads "1 day ago" for every event. #201 replaced literals with this anchoring, which only works for short-lived processes.

Fix

Converted the module-level recentActivity constant into getRecentActivity(), a function that builds the array (calling minutesAgo fresh) on every call instead of once at import time. Updated all four consumers — the home page, and the contributor/maintainer/sponsor dashboards — to call it at render time instead of importing a frozen array.

Testing

• Added a test that mocks system time, reads the first event's occurredAt, advances the clock two simulated days, calls getRecentActivity() again, and asserts the anchor actually moved forward — this would have failed against the old constant-array version.
• Updated the existing asset-coverage/sanity-ceiling tests to call getRecentActivity().
• Full mock-data.test.ts: 7/7 passing. ContributorDashboardClient.test.tsx: 5/5 passing, zero regressions from the import change.
• tsc --noEmit and eslint clean on all six changed files.

Closes #707

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