Add datestamp to backup filename

[e-t-l]

Please see the appropriate readme section for issue reporting guidelines: https://github.com/Helium314/HeliBoard?tab=readme-ov-file#reporting-issues
tl;dr:

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Is your feature request related to a problem? Please describe.
If multiple backups are made, old ones will be overwritten if new file isn't renamed.

Describe the solution you'd like
The filename "Heliboard_backup.zip" should simply include a datestamp so multiple backups can be easily saved. I.e. "Heliboard_backup_20241029.zip" (for a backup file creates on Oct 29th, 2024).

Use case
Users may want to backup different Heliboard configurations to easily switch between them, test out a configuration, export it to a different profile/device, etc.

Describe alternatives you've considered (if any)
Remembering to manually rename the file to something memorable. Which isn't hard, granted, but this would be a small improvement. Probably easy to implement too; I'll try to submit a PR for it if you like the idea.