Simplify calibration model, add pixel preview, and improve UI

- Replace segment-based calibration with core parameters (leds_top/right/bottom/left);
  segments are now derived at runtime via lookup tables
- Fix clockwise/counterclockwise edge traversal order for all 8 start_position/layout
  combinations (e.g. bottom_left+clockwise now correctly goes up-left first)
- Add pixel layout preview overlay with color-coded edges, LED index labels,
  direction arrows, and start position marker
- Move "Add New Device" form into a modal dialog triggered by "+" button
- Add display index selector to device settings modal
- Migrate from requirements.txt to pyproject.toml for dependency management
- Update Dockerfile and docs to use `pip install .`

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/style.css

@@ -164,6 +164,10 @@ section {
     gap: 20px;
 }
 
+.devices-grid > .loading {
+    grid-column: 1 / -1;
+}
+
 .card {
     background: var(--card-bg);
     border: 1px solid var(--border-color);
@@ -487,11 +491,15 @@ section {
     width: 100%;
 }
 
-.add-device-section {
-    background: var(--card-bg);
-    border: 1px solid var(--border-color);
-    border-radius: 8px;
-    padding: 20px;
+.section-header {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    margin-bottom: 15px;
+}
+
+.section-header h2 {
+    margin-bottom: 0;
 }
 
 .form-group {
@@ -981,3 +989,92 @@ input:-webkit-autofill:focus {
         width: 100%;
     }
 }
+
+/* Pixel Layout Preview Overlay */
+.pixel-preview-overlay {
+    position: fixed;
+    top: 0;
+    left: 0;
+    width: 100%;
+    height: 100%;
+    background: #111111;
+    z-index: 3000;
+    display: flex;
+    flex-direction: column;
+    animation: fadeIn 0.2s ease-out;
+}
+
+.pixel-preview-header {
+    display: flex;
+    align-items: center;
+    padding: 12px 20px;
+    background: rgba(0, 0, 0, 0.6);
+    border-bottom: 1px solid #333;
+    flex-shrink: 0;
+}
+
+.pixel-preview-title {
+    font-size: 1.1rem;
+    font-weight: 600;
+    color: #e0e0e0;
+}
+
+.pixel-preview-device-name {
+    font-size: 0.9rem;
+    color: #999;
+    margin-left: 12px;
+}
+
+.pixel-preview-close {
+    margin-left: auto;
+    background: none;
+    border: 1px solid #555;
+    color: #e0e0e0;
+    font-size: 1.2rem;
+    width: 36px;
+    height: 36px;
+    border-radius: 4px;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background 0.2s, border-color 0.2s;
+}
+
+.pixel-preview-close:hover {
+    background: rgba(244, 67, 54, 0.3);
+    border-color: #f44336;
+}
+
+#pixel-preview-canvas {
+    flex: 1;
+    width: 100%;
+    min-height: 0;
+}
+
+.pixel-preview-legend {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 24px;
+    padding: 10px 20px;
+    background: rgba(0, 0, 0, 0.6);
+    border-top: 1px solid #333;
+    flex-shrink: 0;
+    flex-wrap: wrap;
+}
+
+.pixel-preview-legend-item {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    font-size: 0.85rem;
+    color: #ccc;
+}
+
+.pixel-preview-legend-swatch {
+    width: 14px;
+    height: 14px;
+    border-radius: 3px;
+    border: 1px solid rgba(255, 255, 255, 0.3);
+}