Meeting/Committee edit: Save button active without a change done

Describe the bug
Currently in several setting menus the save button is active, even if no changes are done.

Affected setting menus:

• Meeting edit mask
• Committee edit mask

How to Reproduce

1. open one of these setting menus

Expected behavior
The Save button should only be clickable if changes are done.

Context
This is caused by the base-form-field-control.ts which defaults to marking inputs as touched and dirty when initializing the component.

A clean fix should be implemented for this that includes

1. Renaming BaseFormFieldControlComponent<T> to BaseLegacyFormFieldControlComponent<T>
2. Creating BaseFormFieldControlComponent<T> fulfilling the interface for signal form controls (https://angular.dev/guide/forms/signals/custom-controls)
3. Either do the same for BaseSearchSelectorComponent or migrate all dependents
4. Make BaseSearchSelectorComponent more flexible. Identifiable on the Selectable type is too restrictive as it prevents us from using string keys.
5. Update at least AccountSearchSelectorComponent to use new underlying BaseFormFieldControlComponent

Start by reading base-form-field-control.ts and the Angular signal form custom-controls guidance linked in the issue. Trace BaseSearchSelectorComponent and AccountSearchSelectorComponent to identify affected dependents. Done means the meeting and committee Save buttons stay inactive until changes are made, while the requested legacy rename, signal-control implementation, selector flexibility, and AccountSearchSelector migration are complete.