Add skip LEDs feature with physical resampling and per-edge tick labels

Skip LEDs at the start/end of the strip are blacked out while the full
screen perimeter is resampled onto the remaining active LEDs using linear
interpolation. Calibration canvas tick labels show per-edge display
ranges clipped to the active LED range. Moved LED offset control from
inline overlay to a dedicated form row alongside the new skip inputs.

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

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

@@ -176,6 +176,7 @@
   "calibration.tip.span": "Drag green bars to adjust coverage span",
   "calibration.tip.test": "Click an edge to toggle test LEDs",
   "calibration.tip.toggle_inputs": "Click total LED count to toggle edge inputs",
+  "calibration.tip.skip_leds": "Skip LEDs at the start or end of the strip — skipped LEDs stay off",
   "calibration.tutorial.start": "Start tutorial",
   "calibration.start_position": "Starting Position:",
   "calibration.position.bottom_left": "Bottom Left",
@@ -189,6 +190,12 @@
   "calibration.leds.right": "Right LEDs:",
   "calibration.leds.bottom": "Bottom LEDs:",
   "calibration.leds.left": "Left LEDs:",
+  "calibration.offset": "LED Offset:",
+  "calibration.offset.hint": "Distance from physical LED 0 to the start corner (along strip direction)",
+  "calibration.skip_start": "Skip LEDs (Start):",
+  "calibration.skip_start.hint": "Number of LEDs to turn off at the beginning of the strip (0 = none)",
+  "calibration.skip_end": "Skip LEDs (End):",
+  "calibration.skip_end.hint": "Number of LEDs to turn off at the end of the strip (0 = none)",
   "calibration.button.cancel": "Cancel",
   "calibration.button.save": "Save",
   "calibration.saved": "Calibration saved successfully",