Update weekly calendar view to restrict invalid start

[MatoPlus]

Ticket link

Closes #614

Implementation description

• Enhancing the logic I removed in https://github.com/uwblueprint/sistering/pull/578/files#diff-1419dbfb9a8576e06b1e14356bb91202a213dcc954255f30c3019d8b0a7a9c96L128
• we should only specify valid start for non-recurring schedules or schedules with start/end date in the same week. This basic guard is sufficient although it does not consider more specific cases like 2 week posting for 1 month recurring event (which actually should have a start guard)

Steps to test

1. As a super admin, login and go to create new posting page
2. You should not be able to select shifts on days before the start date (we still allow it for recurring events lasting over than 1 week to allow uneven schedules)

What should reviewers focus on?

• Correctness

Checklist

• [x] My PR name is descriptive and in imperative tense
• [x] My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• [x] I have run the appropriate linter(s)
• [x] I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit edbcef3):

https://sistering-dev--pr638-rickson-schedule-shi-ke93pmhv.web.app

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[MatoPlus]

Tested in preview, seems like some days are missing in the calendar when startDate and endDate aren't on the same week for None recurrence frequency e.g. Can't see Sunday or Monday

I don't think there's any actionable item to follow on this edge case within our time frame (eg: full month calednar). As discussed, we can probably ignore it?