Avatar and logo uploads now work reliably

SVG and image uploads for user avatars, org logos, and branch logos now succeed without 403 errors.

Bug Fixes

Uploads of profile pictures and organizational branding assets (SVG, PNG, JPEG, WebP, GIF) were failing with 403 signature errors. The storage layer was using AWS SigV4 signing through Supabase's S3-compatibility proxy, which doesn't handle custom Content-Type headers reliably.

Now using Supabase Storage's native REST upload endpoint with bearer authentication, eliminating the SigV4 fragility and enabling all image types to upload correctly.