fix(docs): broken appLogoUrl example URL returns 404 in multiple SDK code examples

[ygd58]

Bug

Multiple code examples use https://base.org/logo.png as the appLogoUrl parameter, but this URL returns 404.

Verified:

curl -sI https://base.org/logo.png
HTTP/2 404

Developers copying these examples will see a broken logo in their Base Account SDK popup.

Affected files (6):

• docs/base-account/improve-ux/sponsor-gas/paymasters.mdx
• docs/base-account/improve-ux/spend-permissions.mdx
• docs/base-account/improve-ux/sub-accounts.mdx
• docs/base-account/improve-ux/batch-transactions.mdx
• docs/base-account/quickstart/web.mdx
• docs/base-account/quickstart/web-react.mdx

Proposed fix:
Replace with a working URL or add a comment noting developers should use their own logo URL.