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

[st3iny]

Follow-up to #5891
Reverts #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 fix: delay initializing the resize observer until FC is ready #5995).
3. Private widgets in a Talk room (internal link).
4. Public widgets in a Talk room (public link share).

[codecov]

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 ⚠️

Additional details and impacted files

@@             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	Coverage Δ
javascript	15.18% <0.00%> (+0.01%)	⬆️
php	58.46% <ø> (ø)

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[hamza221]

Doesn't break Widgets ✅

[st3iny]

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