[2.x] feat: convert uploaded logos to WebP, support animated GIF

[imorland]

Changes proposed in this pull request:

Converts forum logo uploads to WebP (matching the approach used for avatars in #4431), and adds support for animated GIF logos.

• Static logos → WebP
• Animated logos → GIF (preserving animation)
• UploadImageController default extension updated from png to webp
• UploadLogoDarkModeController inherits this behaviour automatically

Favicon upload is unaffected — it explicitly sets its own extension and .ico passthrough is preserved.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.