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/static/css/base.css

@@ -0,0 +1,61 @@
+* {
+    margin: 0;
+    padding: 0;
+    box-sizing: border-box;
+}
+
+:root {
+    --primary-color: #4CAF50;
+    --danger-color: #f44336;
+    --warning-color: #ff9800;
+    --info-color: #2196F3;
+}
+
+/* Dark theme (default) */
+[data-theme="dark"] {
+    --bg-color: #1a1a1a;
+    --card-bg: #2d2d2d;
+    --text-color: #e0e0e0;
+    --border-color: #404040;
+    --display-badge-bg: rgba(0, 0, 0, 0.4);
+    color-scheme: dark;
+}
+
+/* Light theme */
+[data-theme="light"] {
+    --bg-color: #f5f5f5;
+    --card-bg: #ffffff;
+    --text-color: #333333;
+    --border-color: #e0e0e0;
+    --display-badge-bg: rgba(255, 255, 255, 0.85);
+    color-scheme: light;
+}
+
+/* Default to dark theme */
+body {
+    background: var(--bg-color);
+    color: var(--text-color);
+}
+
+html {
+    background: var(--bg-color);
+    overflow-y: scroll;
+}
+
+body {
+    font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
+    background: var(--bg-color);
+    color: var(--text-color);
+    line-height: 1.6;
+}
+
+body.modal-open {
+    position: fixed;
+    width: 100%;
+}
+
+.container {
+    max-width: 1200px;
+    margin: 0 auto;
+    padding: 20px;
+}