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

## 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.