Fix radial gradient centerpoint drag drift (#126)

* Initial plan

* Fix gradient centerpoint drag drift by correcting pixel-to-percentage conversion

Co-authored-by: argyleink <1134620+argyleink@users.noreply.github.com>

* Revert changes to RadialOverlay and ConicOverlay drag calculations

Co-authored-by: argyleink <1134620+argyleink@users.noreply.github.com>

* Restore improved radial gradient position drag calculation

Co-authored-by: argyleink <1134620+argyleink@users.noreply.github.com>

* Fix radial gradient position drag drift using absolute cursor positioning

Co-authored-by: argyleink <1134620+argyleink@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: argyleink <1134620+argyleink@users.noreply.github.com>

Author: Copilot

src/components/RadialOverlay.svelte

@@ -316,10 +316,16 @@
         }
       }
       else if (dragulaState.moving) {
-        let wpercent = (size.w / 2) / 100
-        let hpercent = (size.h / 2) / 100
-        dragulaState.left += e.movementX / wpercent
-        dragulaState.top += e.movementY / hpercent
+        // Use absolute cursor position relative to the preview element
+        // to avoid drift from accumulated movementX/Y rounding errors
+        const previewEl = node.closest('.preview')
+        if (previewEl) {
+          const rect = previewEl.getBoundingClientRect()
+          const x = ((e.clientX - rect.left) / rect.width) * 100
+          const y = ((e.clientY - rect.top) / rect.height) * 100
+          dragulaState.left = x
+          dragulaState.top = y
+        }
 
         $radial_position.x = Math.round(dragulaState.left)
         $radial_position.y = Math.round(dragulaState.top)