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]>

src/smarttools/web/templates/pages/index.html

@@ -22,7 +22,7 @@
         <!-- Install Command -->
         <div class="mt-10 max-w-xl mx-auto">
             <div class="flex items-center bg-gray-100 rounded-lg p-4 group">
-                <span class="text-gray-400 mr-2 select-none">$</span>
+                <span class="text-gray-600 mr-2 select-none" aria-hidden="true">$</span>
                 <code id="install-command" class="flex-1 text-gray-800 font-mono text-sm md:text-base text-left">pip install smarttools && smarttools init</code>
                 <button type="button"
                         onclick="copyInstallCommand()"
@@ -45,7 +45,7 @@
                 Get Started
             </a>
             <a href="{{ url_for('web.tutorials') }}"
-               class="w-full sm:w-auto inline-flex justify-center items-center px-8 py-4 text-lg font-medium text-cyan-600 border-2 border-cyan-500 rounded-md hover:bg-cyan-50 transition-colors">
+               class="w-full sm:w-auto inline-flex justify-center items-center px-8 py-4 text-lg font-medium text-cyan-700 border-2 border-cyan-600 rounded-md hover:bg-cyan-50 transition-colors">
                 View Tutorials
             </a>
         </div>