fix: compute StatCard trend from history data instead of hardcoded literals (#92)

[rafaio1]

Fixes #92.

• Add computeTrend(history) helper to utils.ts that computes week-over-week percentage change from the last two data points, returning undefined when fewer than 2 points exist or the prior value is zero (avoiding division-by-zero and meaningless trends).
• Replace all three hardcoded trend literals (12, 8, 18) in contributor and sponsor dashboards with computeTrend() calls against the same history arrays already passed as sparklines (contributorEarningsHistory, contributorSparkline, sponsorSpendHistory).
• Trend now correctly renders negative values with the down-arrow/rose styling when the most recent period is lower than the prior — a path previously unexercised since all literals were positive.
• When live backend history data becomes available (per Dashboard charts, activity feeds, and spend-by-repo panels are permanently hardcoded mock data, regardless of the Live data badge #73), these call sites will naturally compute from real data by swapping the mock array for the live one — no structural change needed.
• TypeScript compiles cleanly with npx tsc --noEmit.

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