feat: DATEV Export (EXTF v5.0) + iCal Fristen feed + UI buttons
DATEV Export:
- Real DATEV EXTF Buchungsstapel format (v700, semicolon-separated)
- Header line with metadata (consultant, client, year, currency)
- Data rows: Soll, Haben, Betrag, Belegdatum, Text, Belegnr, USt%
- Also supports simple CSV format
- Audit-logged
- Download button in SKR04/Buchführung tab
iCal Fristen Feed:
- Real .ics calendar feed (RFC 5545 compliant)
- Aggregates ALL deadlines: UStVA, contracts, licenses, assets, projects, invoices
- 23 events from 6 modules
- Subscribe via webcal:// URL in Apple/Google/Outlook Calendar
- Download button in SKR04/Buchführung tab
0 TypeScript errors, ESLint clean
Z User
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