Feature Request: send GIFs from Beeper

Is your feature request related to a problem? Please describe

GIFs cannot be sent from the Beeper Desktop client. The screenshot is at the top of the issue.

Describe the solution you'd like

GIFs sent from Beeper reach LINE as GIFs. Check how each GIF looks on LINE and on Beeper Desktop and mobile, in all of these cases:

• 1:1 chats and groups
• business accounts
• the LINE setting that plays GIFs by default, on and off
• chats without end-to-end encryption. LINE turned it on for every user. But a very old group with inactive members, or a 1:1 with an inactive user, can still be unencrypted.

Test every case with a sender and a receiver. Two LINE accounts are highly recommended.

Describe alternatives you've considered

There were several attempts, and none was merged:

• feat: add GIF support for sending and receiving #98
• Add GIF support #141
• feat: ✨ fully support sending and receiving GIFs #175
• feat: advertise raw GIF support to Beeper Desktop #235
• GIF, sticker and emoji support with media #246 by formeo14

I tried to do everything in one go and did not deliver it. I lost access to the second account, had idle time, lost the context, and redid it a few times. That is why the history below is dense.

Additional context

Do not underestimate this topic. Split it into smaller issues before you start. Have fun 😅

#284 takes the GIF part of #246 and matches it to a capture of the LINE Chrome Extension.