feat: allow user to use local file as background (@fehmer, @byseif21)

[fehmer]

Allow the user to use a local file as custom background without uploading it to the server.

Based on @byseif21 work in #6630, thanks!

[byseif21]

hey @fehmer thanks and yah very nice , just don't we forget to update the setting description too. or you think the aria-label enough ?

[byseif21]

also didn't we still need to add a file size check before storing?

[fehmer]

hi @byseif21 , thanks for your comments. I updated the description and icon to match the same as "open file" in custom text settings. An upload icon might be confusing.

I don't think we need a file size check, the limit for idb is like 20gig or 10% of the users disc space, so we should be fine.

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