As user I want to be able to maintain the calendars centrally, so I don’t need to define the calendar for one person several times in different charts.

Just a bunch of thoughts for further discussion

Requirements:
inheritance, to allow define company/country holidays once and other calendars to extend from it. Otherwise users will have to set holidays in each resource calendar, like it is now.
use global resource calendar (new functionality)
- ADD
- EDIT
- DELETE
use chart resource calendar (only for the chart, existing functionality)

Who has access to global calendars for editing? Add groups list in permissions tab in admin area?
Global means for whole company, or global for user? e.g. can user1 see calendars of user2?
If global for company, who can delete or edit? any who has permissions to global, or only creator?

To consider, but not sure if can be implemented at all
see how much resource is loaded according to global calendar (to use resource in multiple charts)

Possible implementation:
in resource calendar add dialog, after resource selected, we could check if user has global calendar. If yes, we could show option to use global or chart. If no global calendar - should we ask if user wants to create global?

Only for DEVs below
Possible change:
If calendar is global, don't set “gantt_id”
If we don’t set gantt_id, we must know to which host it belongs. So add “host_id” to calendars table. Do we have to set host_id on chart-only calendar?
If we allow global-per-user calendars, we need separate field in calendars table to determine calendar type, like
- “global” - per company, selected by “host_id” (gantt_id is null)
- “user“ - per user, selected by “created_by”
- “chart“ - current behavior, it has “gantt_id“

We could try to detect type based on data (like if host_id is set then it is host-wide) but this may become hard to understand like our settings table. Separate option is much more clear in my opinion.

Structure:

No calendars in app admin settings, only in charts.
They have global calendars for all users. A calendar can “extend” another one, overriding some day (multiple levels of inheritance allowed). As I understand, global calendars can be edited by all users who can open chart settings. Chart has own calendar (holidays greyed out according to it). Resources also have calendar in chart settings (one for all). If needed, resource calendar for specific resource can be selected from resources panel, overriding the default one from chart settings. Also vacations for specific resource can be added from resource panel.