Backdate visibility start when sharing an aircraft

Problem:

When you grant a user access to an aircraft, they only see flights that occur after they accept the invite and visibility is assigned. Existing or in-progress flights are invisible to them, even when access is operationally needed immediately.

Desired outcome:

The granter can specify a visibility start date/time when granting aircraft access — defaulting to now, but back-datable to include earlier flights when needed.

Proposed change:

Add a "visibility start" field to the share/grant dialog. Apply to both new invite acceptance and grants to existing users. Surface the chosen start in the audit trail and in the recipient's view of their access.

Example:

During a search-and-rescue tasking, an aircraft is already airborne when external responders (e.g. police) are invited. Today they cannot see the in-progress flight until they accept and a new flight starts. Backdated visibility lets the granter include the active flight immediately.