Save calendar view presets (calendar sets)

Is your feature request related to a problem? Please describe.

When working with many calendars, it is tedious to constantly toggle them on and off depending on the context (e.g. private vs. work vs. project). There is no quick way to save and restore a preferred set of visible calendars.

Describe the solution you'd like

Introduce calendar view presets (or "calendar sets"), allowing users to:

• Save the current selection of visible calendars as a named preset
• Quickly switch between presets with one click
• Manage presets (rename, delete, update selection)
• Optionally share presets with other users/groups

Describe alternatives you've considered

• Manually toggling calendars on/off in the sidebar (time-consuming and error-prone)
• Publishing multiple calendars in one shared link (#708, #806) → only useful for external viewers, not for daily user workflows
• Muting calendars (#5951) → helps temporarily, but doesn’t solve the need for switching between multiple contexts

Additional context

Example use cases:

• A pastor in a parish wants to quickly switch between “all room bookings”, “staff meetings only”, and “public holidays”.
• A teacher may want “class schedule only” vs. “all private + school calendars”.
• A project manager wants a preset for “project team calendars” and one for “company-wide events”.

Start by reviewing the existing calendar visibility controls and the related issues #708, #806, and #5951. Clarify whether presets are local or shared, then define the storage, management, and switching behavior before identifying the relevant frontend and backend entry points. Done means named presets can save, restore, rename, update, and delete calendar selections.