PDF Invoices & Dark Mode

Added

• PDF invoice generation — Professional A4 layout with SpeyBooks branding
• User registration — Public signup with automatic organisation provisioning
• Dark mode — Light, Dark, and System theme toggle with persistence
• CSV export — All 6 report types exportable
• Invoice email — Send invoices via AWS SES with HTML template
• Domain setup — Migrated to app.speybooks.com with verified DKIM, SPF, DMARC