add file storage in Supabase

[WitchDevelops]

https://supabase.com/docs/guides/storage/buckets/fundamentals

• create bucket in Supabase
• set up generated url as pet image
• add backend function for deleting existing image
• will need frontend dropdown in the pet card component to test if it works
• when it does, create a backend function for image upload using similar logic
• add image uploader to pet form (?)
• add "gallery" to pet tabs
• use image uploader component there - with preview?