Fix referrer_url

[jguo144]

Description

When Cloudflare challenge are enabled, the browser's referrer chain is broken. A user navigating to the contact page can be intercepted by Cloudflare. After passing the challenge, referrer becomes /contact?__cf_chl_tk=... instead of the previously visited URL.

• Add referrer_url as a query parameter to the contact form link
• In the form() GET handler, the referrer is now resolved server-side with the explicit referrer_url query parameter
• Added clean_referrer_url() function that strips any query parameter starting with _cf_chl from a URL, and returns empty string if the cleaned URL is just the contact page
• The JS now only populates the referrer field if the server didn't provide a value

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca