Show and select the configured default when editing quotas #3718

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

When editing an existing quota, administrators cannot see or select the configured default quota. They have to know its value and enter it manually.

Describe the solution you'd like

Make the configured default quota available to clients. In the quota editor, administrators should see a "Default" option with its effective value and be able to select it to apply the default to an existing space.

Keep unlimited (quota.total = 0) distinct from a finite default, and enforce the existing quota permissions and limits.

Describe alternatives you've considered

Expose the default value so clients can use the existing numeric quota update, or provide an explicit operation to apply the server default.

Additional context

Document which defaults apply to project spaces and personal spaces so clients can show the correct value in each quota editor.