feat: add browser-only personal backgrounds

[GaryLauLGY]

Summary

• persist user-uploaded JPG, PNG, and WebP backgrounds in browser IndexedDB only
• restore personal backgrounds after refresh without adding any upload API or server storage
• add per-image deletion, privacy copy, 20 MB validation, and object URL cleanup
• keep personal backgrounds separate from shared PostgreSQL backgrounds

Verification

• browser upload selected the image immediately
• refresh restored the personal image
• Network inspection showed only a local blob: GET and no POST/PUT/PATCH request
• deletion confirmation and persisted deletion verified
• 390×844 viewport exposes upload, personal image, and delete controls
• pnpm test (2/2)
• pnpm lint
• pnpm api:build
• pnpm test:server (16/16)
• pnpm deploy:worker:dry
• git diff --check