Design an "upgrade mode" interface

Context: !90 (comment 2394956033)

When the servers goes down to upgrade, frontend needs to have a user-friendly interim interface, indicating that the Whitebox upgrade/restart is in progress, and that it will be back shortly.

An already loaded app may continue to work (in a limited capacity) [1], e.g. to let the user continue writing the note that they are writing somewhere, but considering that federated components will not be available during backend downtime, a newly loaded page should probably be just a full screen "upgrade mode" message.

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[1] We need to figure out how to draw a clear boundary here of what should work and what shouldn't, write guidelines how (although we can't enforce it on plugin developers), otherwise the UX during this period is gonna come with a lot of ERROR XYZ messages.