feat: add COOKIE_DOMAIN support for cross-subdomain sharing

[monkeyWie]

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[augmentcode]

🤖 Augment PR Summary

Summary: Adds configurable cookie domain support so auth/session cookies can be shared across subdomains.

Changes:

• Passes a new COOKIE_DOMAIN secret through the GitHub Actions deploy workflow.
• Extends the Cloudflare deployment script to inject COOKIE_DOMAIN into production Wrangler vars.
• Wires COOKIE_DOMAIN into the server’s auth configuration and exposes it via AuthConfig.cookieDomain.
• Configures Better Auth cookie attributes to include the configured cookie domain when present.
• Documents the new variable in wrangler.toml as an optional setting.

Technical Notes: When set, the cookie domain is applied at auth initialization time; leaving it unset preserves current single-domain cookie behavior.

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[augmentcode]

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