feat: Allow shared calendars as appointment conflict calendars

[SebastianKrupinski]

Resolves: #3786
Requires: nextcloud/server#48621

Summary

Adjusted "Additional calendars to check for conflicts" selection logic to list all users calendars, personal and shared to be selectable for conflicts.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 23.54%. Comparing base (df34ac0) to head (562e828).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/components/AppointmentConfigModal.vue	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6411   +/-   ##
=========================================
  Coverage     23.54%   23.54%           
  Complexity      454      454           
=========================================
  Files           247      247           
  Lines         11701    11701           
  Branches       2202     2203    +1     
=========================================
  Hits           2755     2755           
  Misses         8626     8626           
  Partials        320      320           

Flag	Coverage Δ
javascript	15.07% <0.00%> (ø)
php	59.53% <ø> (ø)

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