fix: add frontend/src/lib/ to git (was ignored by /lib/ pattern)

The root .gitignore had `lib/` which matched frontend/src/lib/,
causing Docker build to fail with "Module not found: Can't resolve
'@/lib/utils'" and '@/lib/i18n'.

Changed to `/lib/` so it only ignores the Python lib at repo root.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/pricing.ts

@@ -0,0 +1,6 @@
+export const ANNUAL_DISCOUNT_PERCENT = 20;
+export const YEARLY_DISCOUNT_FACTOR = (100 - ANNUAL_DISCOUNT_PERCENT) / 100;
+
+export function computeYearlyFromMonthly(monthly: number): number {
+  return Number((monthly * 12 * YEARLY_DISCOUNT_FACTOR).toFixed(2));
+}