Normalize zero-duration external calendar events

[martin-forge]

Summary

• Clamp zero-duration timed external calendar events to a minimal positive display duration before passing them to FullCalendar.
• Preserve the original provider event data in extendedProps for context menus and debugging.
• Preserve UTC and explicit-offset formatting when normalizing zero-duration ranges, while leaving all-day and non-zero timed events unchanged.

Closes #1823.

Notes

This PR is independent of #1802 and is based on current upstream main.

Validation

• TZ=UTC npm test -- tests/unit/issues/issue-1823-zero-duration-google-calendar-list-duplication.test.ts --runInBand
• npm run docs:check
• npm run i18n:sync
• npm run lint
• npm run typecheck
• TZ=UTC npm test -- --runInBand
• npm run build:test
• obsidian plugin:reload id=tasknotes vault=test