Improve planned maintenance experience for end-users

### User Story
As a GC Forms end-user (form filler),
I want clear, proactive communication when the system is undergoing maintenance/planned downtime,
So that I understand what’s happening and do not experience unexpected errors or data loss.

### Description:

There might be periods of time where we require planned downtime. Currently, end-users have no awareness of when this could happen and no warning while they are in the middle of filling out their forms.

If we keep the system live:
- Users will see error states
- Sev-1 incidents may be triggered
- Form submissions could fail unpredictably

We do not currently have:
- A maintenance banner system for end-users
- A maintenance mode / modal that blocks access that can be set for a given period of time

### How Might We...
How might we proactively communicate scheduled downtime to end-users?
How might we create a reusable maintenance mode capability for future down-time?