feat: UX overhaul — multi-year nav, professional invoices, filters, sorting

UX Navigation:
- Month selector now covers rolling 24 months (multi-year)
- Partition filter now actually filters job data
- Fixed getBillingPeriod Date-object path consistency
- Multi-column sort in job table (JobID, Start, End, Elapsed, Hours, Cost)
- Empty state messages distinguish 'no data' from 'error'
- Country field changed from 5-item dropdown to free text input
- Form validation on Institution Profile required fields

Professional Invoices:
- Complete invoice.py redesign with reportlab
- Dark navy header, alternating row colors, Helvetica typography
- Institution logo support (base64 from profile)
- Proper billing/remittance address blocks
- Line-item table with hours, rate, amount columns
- Subtotal, discount, total calculation
- Payment terms (configurable days), bank details from profile
- Page numbers, footer with generation timestamp
- Due date auto-calculated from paymentTermsDays setting

Author: Sqoia Dev Agent