Fix profile background cache busting

[cmyui]

Summary

• append the profile background row timestamp to rendered image/GIF URLs so cached assets are invalidated after updates
• preserve a type-0 profile background row and monotonically bump the version to handle rapid None -> upload flows
• add local GIF preview handling on the profile background settings page

Fixes #261.

Testing

• go test ./app/handlers/profiles/...
• node --check web/src/js/akatsuki_src.js
• go run /tmp/check_hanayo_templates.go
• go test ./... currently fails in unrelated packages: internal/bbcode panics in parseTwitch, and internal/csrf/cieca aborts on macOS with missing LC_UUID.