🛡️ Sentinel: [HIGH] Robust URL validation for announcement links

[one-more-refactor]

🛡️ Sentinel: Fix URL protocol validation in announcement links

• 🚨 Severity: HIGH
• 💡 Vulnerability: Simple prefix string matching for URL validation allowed potential XSS bypasses using ASCII control characters or null bytes (e.g. java\x00script:).
• 🎯 Impact: An admin or stored payload could introduce malicious executable script URLs in user announcement links.
• 🔧 Fix: Extracted validateUrl() to sanitize URLs by removing ASCII control characters and null bytes (/[\x00-\x1F\x7F]/g), validating structure with the WHATWG URL constructor, and verifying allowed protocols (http:, https:, or same-origin / relative paths while rejecting // protocol-relative links).
• ✅ Verification: Added unit tests in src/main.test.ts covering valid/invalid URLs, obfuscations, and forbidden schemes. All unit tests, svelte-check diagnostics, and Vite production builds pass.

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PR created automatically by Jules for task 5645076397670662031 started by @one-more-refactor

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