Add LED device abstraction layer for multi-controller support

Introduce abstract LEDClient base class with factory pattern so new
LED controller types can plug in alongside WLED. ProcessorManager is
now fully type-agnostic — all device-specific logic (health checks,
state snapshot/restore, fast send) lives behind the LEDClient interface.

- New led_client.py: LEDClient ABC, DeviceHealth, factory functions
- WLEDClient inherits LEDClient, encapsulates WLED health checks and state management
- device_type field on Device storage model (defaults to "wled")
- Rename target_type "wled" → "led" with backward-compat migration
- Frontend: "WLED" tab → "LED", device type badge, type selector in
  add-device modal, device type shown in target device dropdown
- All wled_* API fields renamed to device_* for generic naming

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

server/src/wled_controller/static/style.css

@@ -343,6 +343,15 @@ section {
     gap: 4px;
 }
 
+.device-type-badge {
+    font-size: 10px;
+    font-weight: 700;
+    padding: 1px 6px;
+    border-radius: 3px;
+    background: rgba(255, 255, 255, 0.15);
+    letter-spacing: 0.5px;
+}
+
 .channel-indicator {
     display: inline-flex;
     gap: 2px;
@@ -680,7 +689,14 @@ select {
     color: var(--text-color);
     font-size: 1rem;
     font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
-    transition: border-color 0.2s, box-shadow 0.2s;
+    transition: border-color 0.2s, box-shadow 0.2s, opacity 0.2s;
+}
+
+input[type="number"]:disabled,
+input[type="password"]:disabled,
+select:disabled {
+    opacity: 0.4;
+    cursor: not-allowed;
 }
 
 input[type="range"] {
@@ -1167,6 +1183,35 @@ input:-webkit-autofill:focus {
     font-size: 14px;
 }
 
+.preview-screen-border-width {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    font-size: 13px;
+    font-weight: 600;
+}
+
+.preview-screen-border-width label {
+    white-space: nowrap;
+}
+
+.preview-screen-border-width input {
+    width: 52px;
+    padding: 2px 4px;
+    font-size: 12px;
+    border: 1px solid rgba(255, 255, 255, 0.4);
+    border-radius: 3px;
+    background: rgba(255, 255, 255, 0.15);
+    color: white;
+    text-align: center;
+}
+
+.preview-screen-border-width input:focus {
+    outline: none;
+    border-color: rgba(255, 255, 255, 0.7);
+    background: rgba(255, 255, 255, 0.25);
+}
+
 .preview-screen-total {
     font-size: 16px;
     font-weight: 600;