security: harden CSP, add path traversal protection, improve accessibility

Security fixes:
- Remove external CDN sources from CSP (cdn.jsdelivr.net, cdnjs.cloudflare.com)
- Add path validation functions to prevent path traversal attacks
- Block access to sensitive system directories
- Add isPathAccessible() check for file operations

UI/Accessibility fixes:
- Increase tab close button from 16px to 24px for better touch targets
- Add focus-visible styles for keyboard navigation
- Add ARIA labels to all toolbar buttons
- Add aria-hidden="true" to decorative SVG icons
- Add role="tablist" and role="tab" to tab bar
- Fix duplicate font-size declaration in .preview-content

Reports generated:
- Security vulnerability scan (10 findings)
- STRIDE threat model with MITRE ATT&CK mapping
- Comprehensive UI design review (40 issues)

Amit Haridas

.security-hardening/state.json

@@ -0,0 +1,27 @@
+{
+  "target": "MarkdownConverter Electron Application",
+  "status": "in_progress",
+  "depth": "comprehensive",
+  "compliance_frameworks": ["owasp"],
+  "current_step": 3,
+  "current_phase": 1,
+  "completed_steps": ["vulnerability-scan", "threat-modeling"],
+  "files_created": ["01-vulnerability-scan.md", "02-threat-model.md"],
+  "started_at": "2026-03-15T00:09:00.000Z",
+  "last_updated": "2026-03-15T00:25:00.000Z",
+  "findings_summary": {
+    "critical": 2,
+    "high": 3,
+    "medium": 3,
+    "low": 2,
+    "total": 10
+  },
+  "fixes_applied": {
+    "csp_external_cdns_removed": true,
+    "path_traversal_protection_added": true,
+    "aria_labels_added": true,
+    "focus_visible_styles_added": true,
+    "tab_close_button_resized": true,
+    "duplicate_font_size_fixed": true
+  }
+}