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/tests/test_api.py

@@ -10,13 +10,11 @@ client = TestClient(app)
 
 
 def test_root_endpoint():
-    """Test root endpoint."""
+    """Test root endpoint returns the HTML dashboard."""
     response = client.get("/")
     assert response.status_code == 200
-    data = response.json()
-    assert data["name"] == "WLED Screen Controller"
-    assert data["version"] == __version__
-    assert "/docs" in data["docs"]
+    assert "text/html" in response.headers["content-type"]
+    assert "LED Grab" in response.text
 
 
 def test_health_check():