Split monolithic index.html and style.css for maintainability

- Extract 15 modals and 3 partials from index.html into Jinja2 templates
  (templates/modals/*.html, templates/partials/*.html)
- Split style.css (3,712 lines) into 11 feature-scoped CSS files under
  static/css/ (base, layout, components, cards, modal, calibration,
  dashboard, streams, patterns, profiles, tutorials)
- Switch root route from FileResponse to Jinja2Templates
- Add jinja2 dependency
- Consolidate duplicate @keyframes spin definition
- Browser receives identical assembled HTML — zero JS changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/templates/partials/image-lightbox.html

@@ -0,0 +1,9 @@
+<!-- Image Lightbox -->
+<div id="image-lightbox" class="lightbox" onclick="closeLightbox(event)">
+    <button class="lightbox-close" onclick="closeLightbox()" title="Close">&#x2715;</button>
+    <button id="lightbox-auto-refresh" class="lightbox-refresh-btn" onclick="toggleKCTestAutoRefresh()" title="Auto-refresh" style="display:none">&#x25B6;</button>
+    <div class="lightbox-content">
+        <img id="lightbox-image" src="" alt="Full size preview">
+        <div id="lightbox-stats" class="lightbox-stats" style="display: none;"></div>
+    </div>
+</div>