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

.ui-design/reviews/review_state.json

@@ -0,0 +1,17 @@
+{
+  "review_id": "full-ui-review_20260315",
+  "target": "src/ (Entire UI Directory)",
+  "focus_areas": ["visual", "usability", "code", "performance"],
+  "context": "comprehensive",
+  "platform": "desktop",
+  "status": "complete",
+  "started_at": "2026-03-15T00:09:00.000Z",
+  "completed_at": "2026-03-15T00:12:00.000Z",
+  "issues_found": 40,
+  "severity_counts": {
+    "critical": 4,
+    "major": 18,
+    "minor": 14,
+    "suggestion": 4
+  }
+}