feat(payroll): live tenant-scoped trend, currency and department charts (#481)

[woahwhattheheck]

Closes #481.

GrantFox OSS / Third Campaign: original contribution and conditional reward claim

/claim #481

I am submitting this as the original @woahwhattheheck author for review and consideration under the advertised GrantFox OSS campaign. Please confirm eligible bounty award/amount and the permitted payout process after acceptance; I am affirmatively claiming authorship and any applicable approved reward, not asserting that funds have been awarded or paid.

Complete real-data chart path

• Adds authenticated employer-only GET /api/payroll/analytics guarded by the verified JWT's organizationId (not user-provided organization IDs), using three PostgreSQL aggregation queries over settled payroll transactions in the trailing 12 calendar months.
• Monthly trend: actual settled payment/bonus amounts for every calendar month, with a selectable single asset denomination; no cross-currency amount addition.
• Currency distribution: counts real completed transactions per currency rather than summing USD, XLM, or other mixed units.
• Department costs: joins employees within the same organization and sums settled amounts by employee department and selected currency; unmatched employees are "Unassigned".
• Adds typed API client using the existing frontend auth token and no fabricated fallback/mocks.
• Adds three responsive Recharts line/pie/bar charts, real API load/error/empty/refetch states, asset selector, theme tokens, explicit chart labels, and screen-reader data tables.
• Connects the sidebar's existing /forecast link to the already existing Forecasting page and embeds the historical analytics underneath its predictive forecast.

Source and verification

Original branch: woahwhattheheck:gpt6/payd481-tenant-scoped-analytics-20261009, source head fce073492234c5a67be2cee45248e4040e42ebc7, exact current upstream base 171c74b454daba241bfb75f36d10a0a3a77a68e5. First-party compare: 2 new commits, 6 intended files, zero inherited fork drift, only frontend/reporting/backend payroll analytics surfaces.

I verified the actual GitHub readback, organization-filtered queries, and source import/route insertion. Local full Vite/backend builds and database integration tests were not executed; no CI-green assertion. Recommend focused cd backend && npm run build and cd frontend && npm run build plus a real organization with a completed payroll record in two currency denominations. This work neither claims sponsor acceptance nor payout receipt.