[Feature suggestion] Pop-up to download assets if they are missing

Description

I recently had a pretty bad first experience when installing RA on openSUSE. I first though it was completely broken, but turns out I was missing the assets, and I had obtain them using the online updater. I understand that assets cannot be included in the package, because they are installed in the users' home directory. If they were in a system directory, then they can't be updated by a user.

But overall, like I said it's really bad UX. Just navigating to the online update menu is difficult, because navigation icons don't show up! And it's really not obvious that you're supposed to do that.

See my reddit post: https://www.reddit.com/r/RetroArch/comments/1ci17iv/opensuse_linux_missing_font_andor_icons/

And I also filed a bug with the distro: https://bugzilla.opensuse.org/show_bug.cgi?id=1223618

Expected behavior

My suggestion is the following: upon startup, check if assets are present. If not, show a pop-up telling the user that they will now by downloaded, and in the future they can by updated using the "online updater" menu. Something like

We couldn't find assets. If you just installed Retroarch, this is normal. Assets will now be downloaded from the Retroarch repositories.

Note: in the future, assets may be updated through the Online Updater menu in Retroarch.

(Proceed / cancel buttons)

Also, maybe use a more friendly fallback font, like the system default "sans-serif" or "system-ui".

Start by tracing RetroArch's startup asset checks and the existing Online Updater menu, then inspect how missing fonts and icons are rendered. Done means a missing-assets startup prompt offers proceed or cancel, explains future updates through the Online Updater, and provides a friendlier fallback font.