Add LED skip start/end, rename standby_interval to keepalive_interval, remove migrations

LED skip: set first N and last M LEDs to black on a target. Color sources
(static, gradient, effect, color cycle) render across only the active
(non-skipped) LEDs. Processor pads with blacks before sending to device.

Rename standby_interval → keepalive_interval across all Python, API
schemas, and JS. from_dict falls back to old key for existing configs.

Remove legacy migration functions (_migrate_devices_to_targets,
_migrate_targets_to_color_strips) and legacy fields from target model.

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

server/src/wled_controller/static/css/modal.css

@@ -211,6 +211,26 @@
     font-style: italic;
 }
 
+.inline-fields {
+    display: flex;
+    gap: 12px;
+}
+
+.inline-field {
+    flex: 1;
+}
+
+.inline-field label {
+    display: block;
+    margin-bottom: 4px;
+    font-size: 0.85rem;
+    color: #aaa;
+}
+
+.inline-field input[type="number"] {
+    width: 100%;
+}
+
 .fps-hint {
     display: block;
     margin-top: 4px;