fix: navigation when clicking on the grid on a custom week/month

[st3iny]

Follow-up to https://github.com/nextcloud/calendar/pull/5891 Reverts https://github.com/nextcloud/calendar/pull/5995

I converted the FullCalendar options to a computed prop again. It still works in private and public widgets. I assume this was just an oversight. This also means that we can get rid of some reactivity hacks as the computed prop is now properly reactive again.

How to reproduce

1. Open the calendar app.
2. Switch to month view.
3. Navigate to the next month using the top left navigation buttons.
4. Click on some day.
5. Observe the grid changing back to the previous month which includes today.

Regression testing

Things that I tested successfully:

1. Main calendar grid view in the app.
2. Resize observer still works (see #5995).
3. Private widgets in a Talk room (internal link).
4. Public widgets in a Talk room (public link share).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 23.54%. Comparing base (4f2caf6) to head (a633a5a). Report is 3 commits behind head on main.

Files	Patch %	Lines
src/components/CalendarGrid.vue	0.00%	14 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5999 +/- ## ============================================ + Coverage 23.51% 23.54% +0.02% Complexity 455 455 ============================================ Files 246 246 Lines 11718 11707 -11 Branches 2154 2142 -12 ============================================ Hits 2756 2756 + Misses 8647 8637 -10 + Partials 315 314 -1 ``` | [Flag](https://app.codecov.io/gh/nextcloud/calendar/pull/5999/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/5999/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.18% <0.00%> (+0.01%)` | :arrow_up: | | [php](https://app.codecov.io/gh/nextcloud/calendar/pull/5999/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `58.46% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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[st3iny]

/backport 209bb44eec28e3ff77dc1d15bda3f6023b3269e4 to stable4.7

[backportbot[bot]]

The backport to stable4.7 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable4.7
git pull origin stable4.7

# Create the new backport branch
git checkout -b backport/5999/stable4.7

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 209bb44e

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/5999/stable4.7

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[st3iny]

Hmm, private widgets were not backported to stable4.7 so we should be good.