[Bug] Agent avatar upload skips the crop/downscale that community icons already get

[w-y-a-t-t]

Summary

When creating (or editing) an agent in Buzz Desktop, uploading a custom avatar image does not resize, downscale, or center-crop it. The raw uploaded file is sent as-is, so a large or non-square photo doesn't get shaped into a proper small square avatar the way it should.

Root cause (traced in code)

useAvatarUpload (desktop/src/features/profile/useAvatarUpload.ts:18,64-65) accepts an optional processImage(file): Promise<string> hook. When provided, it runs the file through that hook before upload; when omitted, it uploads the raw bytes untouched via uploadMediaBytes.

• Community icons already solve this. CommunityIconSettingsCard.tsx:114 passes processImage={downscaleIconToDataUrl} (desktop/src/features/communities/lib/downscaleIcon.ts:11-37), which center-crops to a square and downscales to 128px before encoding to WebP/PNG.
• Agent avatars do not. AgentDefinitionDialog.tsx:770 and AgentInstanceEditDialog.tsx:882 both render <AgentCreationPreview ... /> (which threads processImage down to useAvatarUpload) but neither passes a processImage prop — so agent avatar uploads skip cropping/downscaling entirely.

Expected

Agent avatar upload should reuse the same center-crop + downscale treatment community icons already get, so users can drop in any photo and have it become a properly framed square avatar without needing to pre-edit the image themselves.

Suggested fix

Generalize downscaleIconToDataUrl (it's not community-specific — just takes a File and returns a data URL) into a shared location, and pass it as processImage in AgentDefinitionDialog.tsx and AgentInstanceEditDialog.tsx, same pattern as CommunityIconSettingsCard.tsx.

Duplicate search

Searched open/closed issues for avatar resize, avatar crop, agent avatar, agent icon, resize image, downscale, square image, etc. — no existing issue describes this. Closest are #2366 (icon lost across communities — different bug) and #3089 (proposal for AI-generated avatars — unrelated to manual upload sizing).