fix: preserve sign in formatCurrency, ensure consistency with StatCard

[rindicomfort]

Closes #90

Problem

formatCurrency unconditionally applied Math.abs(), silently discarding negative signs on financial figures. This created an inconsistency with StatCard's internal currency formatter, which correctly preserves signs.

The same negative amount would render as:

• -50 USDC in StatCard (correct)
• 50 USDC in formatCurrency (sign lost)

This is a real, live landmine for any future feature that needs to display negative values such as:

• Net-negative sponsor balances (refunds exceeding deposits)
• Accounting corrections or adjustments
• Budget deltas (e.g., remaining budget after overspending)

Solution

✅ Removed Math.abs() from formatCurrency to preserve negative signs naturally
✅ Added comprehensive JSDoc explaining the sign-preservation behavior and legitimate use cases
✅ Reconciled with StatCard – both formatters now handle negative values identically
✅ Added extensive test coverage to prevent regression

Changes

• src/lib/utils.ts: Removed Math.abs(), added detailed documentation explaining why sign preservation is correct
• src/lib/utils.test.ts: New comprehensive test suite with 36 tests covering sign preservation, locale formatting, edge cases, and all utility functions
• src/components/ui/StatCard.test.tsx: Added 5 new tests confirming negative value handling and consistency with formatCurrency

Testing

All 52 tests pass, including:

• ✅ Sign preservation for negative amounts: formatCurrency(-50, 'USDC') → '-50 USDC'
• ✅ Negative and positive are distinguishable: formatCurrency(-50) !== formatCurrency(50)
• ✅ StatCard and formatCurrency produce identical output for negative values
• ✅ No regressions to existing positive/zero cases across all call sites
• ✅ Edge cases: NaN, Infinity, very large/small negative values
• ✅ Locale formatting preserved: thousand separators, 2 decimal places

Verification

npm test
# All 52 tests pass (36 in utils.test.ts + 16 in StatCard.test.tsx)

Impact

• Consistency: formatCurrency and StatCard now agree on sign-handling – no more contradictory behavior
• Correctness: Financial figures can display negative values correctly throughout the app
• Future-proof: Negative amounts (refunds, corrections, net-negative balances) will render properly when they occur
• Zero regression: All existing positive/zero value use cases continue to work identically

Call Sites Verified

All 15+ call sites across the app continue to work correctly:

• Landing page stats, leaderboard earnings, activity feed
• Dashboard pages (contributor, maintainer, sponsor)
• Bounty cards, milestone pages, reputation pages
• Issue detail pages

No call site currently passes negative values (all mock data is non-negative), so this change is invisible in production today but fixes a latent bug that would surface the moment real negative data appears.

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Ready for merge ✅
All acceptance criteria from issue #90 met, fully tested, zero regressions.

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