Add Sentry error monitoring and fix accessibility issues

Sentry integration:
- Add sentry-sdk[flask] to registry dependencies
- Initialize Sentry with Flask integration (if SENTRY_DSN is set)
- Configure trace/profile sampling rates via environment variables
- Privacy-first: send_default_pii=False

Accessibility fixes (WCAG 2.0 AA compliance):
- Add aria-label to search input
- Add label and aria-label to essential cookies checkbox
- Fix color contrast issues:
  - $ prompt: text-gray-400 → text-gray-600
  - Version numbers: text-gray-400 → text-gray-600
  - Tutorial button: text-cyan-600 → text-cyan-700
  - Step labels: text-indigo-500 → text-indigo-700
  - Footer text: text-gray-500 → text-gray-300 (on dark bg)

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

pyproject.toml

@@ -46,11 +46,13 @@ dev = [
 registry = [
     "Flask>=2.3",
     "argon2-cffi>=21.0",
+    "sentry-sdk[flask]>=1.0",
 ]
 all = [
     "urwid>=2.1.0",
     "Flask>=2.3",
     "argon2-cffi>=21.0",
+    "sentry-sdk[flask]>=1.0",
 ]
 
 [project.scripts]