feat: implement unified DocumentNavigator with lazy loading for all modes

- Add DocumentNavigator UI element for document navigation across viewing and editing modes
- Implement lazy loading approach where control appears immediately but navigation content builds on-demand
- Position controls on left side following UI convention for consistent navigation experience
- Add scroll spy functionality for current section detection
- Include responsive design with mobile auto-hide
- Create comprehensive development guardrails to prevent JavaScript corruption
- Add JavaScript validation tool for syntax error detection

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/validate_js.py

@@ -0,0 +1,161 @@
+#!/usr/bin/env python3
+"""
+JavaScript Validation Tool
+
+Extracts JavaScript from HTML files and validates syntax.
+Detects common issues like "Uncaught SyntaxError: unexpected token"
+"""
+
+import re
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+
+def extract_javascript_from_html(html_file):
+    """Extract all JavaScript code from an HTML file."""
+    try:
+        content = Path(html_file).read_text(encoding='utf-8')
+    except Exception as e:
+        print(f"❌ Failed to read {html_file}: {e}")
+        return []
+
+    # Find all <script> blocks
+    script_pattern = r'<script[^>]*>(.*?)</script>'
+    scripts = re.findall(script_pattern, content, re.DOTALL | re.IGNORECASE)
+
+    # Filter out empty scripts and external script tags
+    js_blocks = []
+    for script in scripts:
+        script = script.strip()
+        if script and not script.startswith('http'):  # Skip external scripts
+            js_blocks.append(script)
+
+    return js_blocks
+
+
+def validate_javascript_syntax(js_code):
+    """Validate JavaScript syntax using Node.js."""
+    try:
+        # Create temporary JS file
+        with tempfile.NamedTemporaryFile(mode='w', suffix='.js', delete=False) as f:
+            f.write(js_code)
+            temp_file = f.name
+
+        # Try to parse with node --check
+        result = subprocess.run(
+            ['node', '--check', temp_file],
+            capture_output=True,
+            text=True
+        )
+
+        # Clean up
+        Path(temp_file).unlink()
+
+        if result.returncode == 0:
+            return True, "✓ Syntax OK"
+        else:
+            return False, f"❌ Syntax Error: {result.stderr.strip()}"
+
+    except FileNotFoundError:
+        # Fallback: Try to detect obvious syntax errors
+        return validate_javascript_basic(js_code)
+    except Exception as e:
+        return False, f"❌ Validation failed: {e}"
+
+
+def validate_javascript_basic(js_code):
+    """Basic JavaScript syntax validation without Node.js."""
+    errors = []
+
+    # Check for common syntax issues
+    if js_code.count('{') != js_code.count('}'):
+        errors.append("Mismatched curly braces")
+
+    if js_code.count('(') != js_code.count(')'):
+        errors.append("Mismatched parentheses")
+
+    if js_code.count('[') != js_code.count(']'):
+        errors.append("Mismatched square brackets")
+
+    # Check for unescaped quotes in strings
+    if re.search(r'["\']\s*["\']\s*["\']\s*["\']', js_code):
+        errors.append("Possible quote escaping issue")
+
+    # Check for Python-style string formatting leftover
+    if '${' in js_code and '"}' in js_code:
+        errors.append("Possible Python string template leftover")
+
+    if errors:
+        return False, f"❌ Potential issues: {', '.join(errors)}"
+    else:
+        return True, "✓ Basic validation passed (Node.js not available)"
+
+
+def validate_html_js(html_file):
+    """Validate all JavaScript in an HTML file."""
+    print(f"🔍 Validating JavaScript in: {html_file}")
+
+    js_blocks = extract_javascript_from_html(html_file)
+
+    if not js_blocks:
+        print("⚠️  No JavaScript found in HTML file")
+        return True
+
+    print(f"📝 Found {len(js_blocks)} JavaScript blocks")
+
+    all_valid = True
+    for i, js_block in enumerate(js_blocks, 1):
+        print(f"\n--- Script Block {i} ({len(js_block)} chars) ---")
+
+        # Show first few lines for context
+        lines = js_block.split('\n')[:3]
+        preview = '\n'.join(lines)
+        if len(lines) < len(js_block.split('\n')):
+            preview += "\n..."
+        print(f"Preview:\n{preview}")
+
+        is_valid, message = validate_javascript_syntax(js_block)
+        print(message)
+
+        if not is_valid:
+            all_valid = False
+            # Show more context around potential error
+            if 'line' in message.lower():
+                try:
+                    line_num = re.search(r'line (\d+)', message, re.IGNORECASE)
+                    if line_num:
+                        line_no = int(line_num.group(1))
+                        lines = js_block.split('\n')
+                        start = max(0, line_no - 3)
+                        end = min(len(lines), line_no + 2)
+                        print("\nContext around error:")
+                        for j in range(start, end):
+                            marker = ">>> " if j == line_no - 1 else "    "
+                            print(f"{marker}{j+1:3}: {lines[j]}")
+                except:
+                    pass
+
+    print(f"\n{'✅' if all_valid else '❌'} Overall result: {'All JavaScript is valid' if all_valid else 'JavaScript validation failed'}")
+    return all_valid
+
+
+def main():
+    if len(sys.argv) != 2:
+        print("Usage: python validate_js.py <html_file>")
+        print("Example: python validate_js.py /tmp/test-edit.html")
+        sys.exit(1)
+
+    html_file = sys.argv[1]
+
+    if not Path(html_file).exists():
+        print(f"❌ File not found: {html_file}")
+        sys.exit(1)
+
+    success = validate_html_js(html_file)
+    sys.exit(0 if success else 1)
+
+
+if __name__ == "__main__":
+    main()