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/test-stream.html

@@ -0,0 +1,23 @@
+<!-- Test Source Modal -->
+<div id="test-stream-modal" class="modal">
+    <div class="modal-content">
+        <div class="modal-header">
+            <h2 data-i18n="streams.test.title">Test Source</h2>
+            <button class="modal-close-btn" onclick="closeTestStreamModal()" title="Close">&#x2715;</button>
+        </div>
+        <div class="modal-body">
+            <div class="form-group">
+                <label for="test-stream-duration">
+                    <span data-i18n="streams.test.duration">Capture Duration (s):</span>
+                    <span id="test-stream-duration-value">5</span>
+                </label>
+                <input type="range" id="test-stream-duration" min="1" max="10" step="1" value="5" oninput="updateStreamTestDuration(this.value)" />
+            </div>
+
+            <button type="button" class="btn btn-primary" onclick="runStreamTest()" style="margin-top: 16px;">
+                <span data-i18n="streams.test.run">🧪 Run</span>
+            </button>
+
+        </div>
+    </div>
+</div>