v2.6.0 — Biannual Billing Cycle

What's Changed

🆕 New Features

• Biannual billing cycle — new option for subscriptions billed every 6 months (twice per year)
• Monthly spending calculated as amount ÷ 6, yearly as amount × 2
• Billing cycle dropdown ordered by frequency: Weekly → Monthly → Quarterly → Biannual → Yearly
• Demo subscription "Car Insurance" added to development seeder

🧪 Tests

• Backend: 79 tests (+4 new)
• Frontend: 160 tests (+8 new)

📝 No Breaking Changes

• No database migration required
• Existing subscriptions unaffected
• Analytics calculations include biannual subscriptions automatically

Full Changelog: v2.5.2...v2.6.0