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/style.css

@@ -1195,40 +1195,6 @@ input:-webkit-autofill:focus {
     gap: 6px;
 }
 
-.offset-control {
-    display: flex;
-    align-items: center;
-    gap: 3px;
-    height: 26px;
-    padding: 0 10px;
-    background: rgba(255, 255, 255, 0.15);
-    border: 1px solid rgba(255, 255, 255, 0.3);
-    border-radius: 12px;
-    font-size: 12px;
-    color: rgba(255, 255, 255, 0.9);
-    box-sizing: border-box;
-    cursor: pointer;
-    user-select: none;
-}
-
-.offset-control input {
-    width: 36px;
-    padding: 0;
-    background: transparent;
-    border: none;
-    color: white;
-    font-size: 12px;
-    text-align: center;
-    outline: none;
-    -moz-appearance: textfield;
-}
-
-.offset-control input::-webkit-outer-spin-button,
-.offset-control input::-webkit-inner-spin-button {
-    -webkit-appearance: none;
-    margin: 0;
-}
-
 .preview-edge {
     position: absolute;
     display: flex;