Fix recurring all-day ICS event end dates

[abbiefalls90]

Summary

• Preserve date-only duration when expanding recurring all-day ICS subscription events
• Prevent later all-day recurrence instances from keeping the original event end date
• Add regression coverage for one-day and multi-day all-day recurrences

Tests

• npm test -- --runTestsByPath tests/unit/issues/issue-ics-recurring-all-day-end-date.test.ts
• npm test -- --runInBand tests/unit/issues/issue-ics-recurring-all-day-end-date.test.ts tests/unit/issues/issue-1626-ics-recurring-events-maxinstances-cap.test.ts tests/unit/issues/issue-1738-outlook-ics-timezone-alias.test.ts tests/unit/utils/icsTimezoneFallback.test.ts
• npm run typecheck
• npm run lint:ts
• npm run build:test

Note: full npm test -- --runInBand still has unrelated existing failures in timezone/date expectation tests in this local environment.