🛡️ Sentinel: Fix URL protocol validation to prevent XSS in announcement links

[one-more-refactor]

This PR addresses a potential XSS vulnerability in announcement link submission.

🚨 Severity: HIGH
💡 Vulnerability: Simple prefix string matching for URL validation can be bypassed using embedded ASCII control characters or null bytes (java\x00script:, java\x09script:) or non-standard relative protocol syntax.
🎯 Impact: Attackers could inject executable javascript: links into user-facing announcement banners.
🔧 Fix: Strip control characters (/[\x00-\x1F\x7F]/g), validate allowed link prefixes (http://, https://, /), and parse with new URL(sanitized, base) to ensure strict http: or https: protocol adherence.
✅ Verification: Created unit tests in src/url.test.ts verifying valid URLs, relative paths, control character stripping, and rejection of dangerous protocols (javascript:, data:, vbscript:). Verified using bun test, bun run check, and bun run build.

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PR created automatically by Jules for task 1720665581646687444 started by @one-more-refactor

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