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/tutorial-overlay.html

@@ -0,0 +1,16 @@
+<!-- Device Tutorial Overlay (viewport-level) -->
+<div id="device-tutorial-overlay" class="tutorial-overlay tutorial-overlay-fixed">
+    <div class="tutorial-backdrop"></div>
+    <div class="tutorial-ring"></div>
+    <div class="tutorial-tooltip">
+        <div class="tutorial-tooltip-header">
+            <span class="tutorial-step-counter"></span>
+            <button class="tutorial-close-btn" onclick="closeTutorial()">&times;</button>
+        </div>
+        <p class="tutorial-tooltip-text"></p>
+        <div class="tutorial-tooltip-nav">
+            <button class="tutorial-prev-btn" onclick="tutorialPrev()">&#8592;</button>
+            <button class="tutorial-next-btn" onclick="tutorialNext()">&#8594;</button>
+        </div>
+    </div>
+</div>