Move overlay toggle into calibration visual editor, add tutorial step

Place the overlay button inside the preview screen as a pill toggle,
add it as a tutorial step that auto-skips in device calibration mode.
Tutorial engine now skips hidden/missing targets in both directions.

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

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

@@ -208,11 +208,13 @@
   "calibration.tip.offset": "Set LED offset — distance from LED 0 to the start corner",
   "calibration.tip.span": "Drag green bars to adjust coverage span",
   "calibration.tip.test": "Click an edge to toggle test LEDs",
+  "calibration.tip.overlay": "Toggle screen overlay to see LED positions and numbering on your monitor",
   "calibration.tip.toggle_inputs": "Click total LED count to toggle edge inputs",
   "calibration.tip.border_width": "How many pixels from the screen edge to sample for LED colors",
   "calibration.tip.skip_leds_start": "Skip LEDs at the start of the strip — skipped LEDs stay off",
   "calibration.tip.skip_leds_end": "Skip LEDs at the end of the strip — skipped LEDs stay off",
   "calibration.tutorial.start": "Start tutorial",
+  "calibration.overlay_toggle": "Overlay",
   "calibration.start_position": "Starting Position:",
   "calibration.position.bottom_left": "Bottom Left",
   "calibration.position.bottom_right": "Bottom Right",