🛡️ Sentinel: Fix potential XSS via announcement link protocol evasion

[one-more-refactor]

🛡️ Sentinel: Fix potential XSS via announcement link protocol evasion

• Severity: HIGH
• Vulnerability: Weak link URL validation relying on string prefix matching, which can be vulnerable to bypasses using control characters, case variations, or protocol scheme tricks (e.g. javascript:, data:).
• Impact: Stored Cross-Site Scripting (XSS) if an operator sets an announcement link to an executable script payload that users click on.
• Fix: Used WHATWG URL constructor with fallback origin to parse relative and absolute links, stripping ASCII control characters ([\x00-\x1F\x7F]) and enforcing http: or https: protocol checks.
• Verification: Added src/main.test.ts unit test suite covering javascript:, data:, vbscript:, null byte injection, and valid HTTP(S) links. Verified via bun test, bun run check, and bun run build.

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PR created automatically by Jules for task 11899933106075306149 started by @one-more-refactor

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