Refactor callsign.js: eliminate XSS vulnerability, modernize codebase

[Copilot]

Complete refactoring addressing security, performance, and code quality issues throughout the library.

Security

• Eliminated XSS vulnerability in searchCallsigns() - replaced innerHTML string manipulation with TreeWalker + DocumentFragment
• Changed innerHTML assignments to textContent where applicable

Performance

• Configuration caching eliminates repeated getElementById() calls
• TreeWalker provides O(n) DOM traversal vs O(n²) innerHTML regex replacement

Code Quality

• ES6+ modernization: const/let, template literals, arrow functions, object shorthand
• Extracted helpers: getScriptElement(), getConfig(), createFlagElement()
• Input validation with descriptive warnings for malformed call signs
• Enhanced ESLint rules: complexity limits, best practices enforcement (20+ new rules)

API Extensions

• New data-css-path attribute for custom stylesheet location
• CSS custom properties for theming:

  call-sign {
    --cs-border-color: #007acc;
    --cs-background-color: #e0f0ff;
  }

Before/After

// Before: Dangerous innerHTML manipulation
static searchCallsigns() {
  let html = document.body.innerHTML;
  while ((match = html.match(SEARCH_REGEX)) !== null) {
    html = html.replace(match[1], '<call-sign>' + match[1] + '</call-sign>');
  }
  document.body.innerHTML = html;  // ⚠️ XSS risk, breaks event listeners
}

// After: Safe DOM manipulation
static searchCallsigns() {
  const walker = document.createTreeWalker(
    document.body,
    NodeFilter.SHOW_TEXT,
    { acceptNode(node) { /* filter logic */ } }
  );
  // ... collect nodes, build fragments, replace safely
}

Statistics: 5 files, +250/-51 lines. Backward compatible.

Original prompt

Go through the entire codebase and suggest refactoring and other improvements

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