Add support for rolling monthly deadlines

[thromel]

Summary

Add support for conferences with rolling monthly deadlines (e.g., VLDB) without requiring separate YAML entries for each month.

Github Issue: #36

Changes

New rolling_deadline YAML schema

Conferences with recurring monthly deadlines can now use a compact configuration:

- name: VLDB
  year: 2026
  rolling_deadline:
    submission_day: 1           # Submit on 1st of each month
    notification_day: 15        # Notification on 15th
    notification_month_offset: 1  # of the next month
    start: "2025-04-01"         # First deadline
    end: "2026-03-01"           # Final deadline
  date: "2026-08-31"
  place: Boston, MA

Code changes

• Add expandRollingDeadlines() function in FetchConferences.js to expand rolling deadlines into individual calendar entries
• Each cycle generates its own deadline and notification date
• Updated VLDB 2025/2026 entries to use the new schema

Benefits

• Cleaner YAML: 1 entry instead of 12 for VLDB
• Calendar displays all monthly deadlines automatically
• Easy to add other conferences with rolling deadlines

Test plan

• VLDB deadlines appear on 1st of each month (Apr 2025 - Mar 2026)
• VLDB notifications appear on 15th of following month
• Other conferences still display correctly
• App compiles without errors