Pwnagotchi Birthday celebrations based on epochs

Purely a fun thing to celebrate when an AI reaches a significant number of epochs. We can't really measure time, so an epoch 'feels' like a more suitable measure of age.

When an pwnagotchi reaches a multiple of a significant number of epochs (say 1000) it's happy face says the following for the next few epochs when it has relatively neutral feelings. Any significant feelings such as seeing friends and finding SSIDs override it so that the user can still see the pwnagotchi is still working on its birthday.

`(*∇*)由 Happy birthday to me! I'm xxxx epochs old`

The face is made smaller (3 figures instead of 4) to accommodate the cake so should still fit.

This has the benefit of reminding the owner that their pwnagotchis age and are always learning :)

Locate the epoch counter, happy-face rendering, and feeling-priority logic first. Confirm how a significant epoch interval should be selected, then implement the birthday message and cake-sized face while preserving stronger feelings; done means the celebration appears for the specified epochs and is overridden by significant events.

Written by the indexing model from the issue text.