Replace deprecated sprite url commands

[kcmartin]

Sweeps the remaining sprite url usages, following the rc44 deprecation ('sprite url' -> 'sprite info' to view, 'sprite config update --url-auth' to change). Both replacement commands verified working on rc44.

Changes across quickstart, working-with-sprites, sprites, and reference/configuration:

• sprite url -> sprite info
• sprite url update --auth public -> sprite config update --url-auth public
• sprite url update --auth default -> sprite config update --url-auth sprite (default was never a valid mode; rc44 modes are sprite and public)
• quickstart: prose fixed to name the sprite auth mode instead of default
• working-with-sprites: URL format corrected to https://<name>-<org>.sprites.app (the org segment was missing)

Not touched: cli/commands.mdx (auto-generated from sprite-env client/docs/, would revert on regen; upstream help text still shows the deprecated commands) and concepts/networking.mdx (the draft is replaced wholesale by #204, which already uses the new commands).

Related: the deprecation string in sprite-env client/commands/url.go says sprite update --url-auth, missing the word "config".

[github-actions]

Preview Deployment

Name	URL
Preview	https://pr-207-superfly-sprites-docs.fly.dev

Commit: 1e47552

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO
/	🟢 97	🟢 100	🟢 96	🟢 100
/quickstart/	🟢 94	🟢 100	🟢 100	🟢 100
/working-with-sprites/	🟢 96	🟢 100	🟢 100	🟢 100

[github-actions]

E2E Test Results

✅ Tests success

Ran against: https://pr-207-superfly-sprites-docs.fly.dev