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/modals/confirm.html

@@ -0,0 +1,16 @@
+<!-- Confirmation Modal -->
+<div id="confirm-modal" class="modal">
+    <div class="modal-content" style="max-width: 450px;">
+        <div class="modal-header">
+            <h2 id="confirm-title">Confirm Action</h2>
+            <button class="modal-close-btn" onclick="closeConfirmModal(false)" title="Close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <p id="confirm-message" class="modal-description"></p>
+        </div>
+        <div class="modal-footer">
+            <button class="btn btn-danger" id="confirm-no-btn" onclick="closeConfirmModal(false)">No</button>
+            <button class="btn btn-secondary" id="confirm-yes-btn" onclick="closeConfirmModal(true)">Yes</button>
+        </div>
+    </div>
+</div>