Show reminders as "muted" when DND is active

[jaspermayone]

Context

PR WITCodingClub/calendar-backend#259 fixes the backend so that the
API returns actual configured reminders even when DND is active. The API now includes a
notifications_disabled flag.

Related backend issue: WITCodingClub/calendar-backend#215

What's needed

The extension should use the new notifications_disabled flag to visually indicate when
reminders are muted.

API Changes

These endpoints now include notifications_disabled: boolean:

• GET /api/meeting_times/:id/preference
• PUT /api/meeting_times/:id/preference
• GET /api/user/events/:term_uid

Suggested UI

When notifications_disabled: true:

• Show reminders grayed out or with reduced opacity
• Add a DND icon or "muted" indicator
• Maybe show a tooltip explaining reminders are temporarily disabled

This helps users understand their reminder settings are preserved but currently not active
due to DND mode.