Remove target segments, use single color strip source per target

Segments are redundant now that the "mapped" CSS type handles spatial
multiplexing internally. Each target now references one color_strip_source_id
instead of an array of segments with start/end/reverse ranges.

Backward compat: existing targets with old segments format are migrated
on load by extracting the first segment's CSS source ID.

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

server/src/wled_controller/static/locales/en.json

@@ -365,15 +365,9 @@
   "targets.device": "Device:",
   "targets.device.hint": "Select the LED device to send data to",
   "targets.device.none": "-- Select a device --",
-  "targets.segments": "Segments:",
   "targets.color_strip_source": "Color Strip Source:",
-  "targets.segments.hint": "Each segment maps a color strip source to a pixel range on the LED strip. Gaps between segments stay black. A single segment with Start=0, End=0 auto-fits to the full strip.",
-  "targets.segments.add": "+ Add Segment",
-  "targets.segment.start": "Start:",
-  "targets.segment.end": "End:",
-  "targets.segment.reverse": "Reverse",
-  "targets.segment.remove": "Remove segment",
-  "targets.no_segments": "No segments",
+  "targets.color_strip_source.hint": "Select the color strip source that provides LED colors for this target",
+  "targets.no_css": "No source",
   "targets.source": "Source:",
   "targets.source.hint": "Which picture source to capture and process",
   "targets.source.none": "-- No source assigned --",