[Follow-up] Configurable raw base for the academic calendar #31

Context

#29 and #30 made the SUSTech Online raw base configurable. The academic
calendar still hard-codes https://raw.githubusercontent.com/dumixthestpd/sustech-calendar/main
in src/calendar/client.ts, so sustech calendar day, sustech tis schedule,
tis ical, and other calendar-derived commands fail on networks that cannot
reach raw.githubusercontent.com.

The follow-up was suggested in #30.

Proposal

• Add SUSTECH_CALENDAR_RAW_BASE, defaulting to the current public repository
  base, so behaviour is unchanged when unset.
• Reuse the HTTPS base validation shared with the online layer.
• Reject non-HTTPS values, values with embedded credentials, and values with a
  query string or fragment, with CALENDAR_RAW_BASE_INVALID.

A PR follows.