🛡️ Sentinel: Fix URL protocol validation and block protocol-relative XSS/redirect bypasses

[one-more-refactor]

• Refactored announcement link URL validation into a reusable validateUrl helper in src/lib/url.ts.
• Strips ASCII control characters and null bytes ([\x00-\x1F\x7F]) before parsing with URL constructor to prevent scheme obfuscation (e.g., java\x00script:).
• Blocks protocol-relative URLs (//evil.com) which previously bypassed clean.startsWith('/') checks to prevent open redirects.
• Restricts allowed URL protocols strictly to http: and https:.
• Added unit tests in src/url_validation.test.ts covering valid URLs, relative paths, protocol-relative bypasses, and XSS payload schemes.

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PR created automatically by Jules for task 8773958636347755903 started by @one-more-refactor

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