refactor(line-selection): extract dx/dy, fix actual 1 px gap

Reviewer-spotted (gemini-code-assist on PR #51):

- Hoist dispX2-dispX1 / dispY2-dispY1 into dx, dy locals; less
  repetition in the perpendicular-vector math.
- Offset was lineStrokeWidth / 2 + 1, which puts the selection stroke
  *centre* 1 px past the body's edge. With a 1 px-wide selection
  stroke that leaves an actual visual gap of only 0.5 px between the
  edges. Bump to +1.5 so the gap matches the comment.

Author: u8array

src/components/Canvas/LineObject.tsx

@@ -222,11 +222,21 @@ export function LineObject({
         // top) keeps the body's difference-blend intact for reverse
         // lines and matches the Illustrator-style stroke selection
         // affordance. Guarded against zero-length (degenerate) lines.
-        const len = Math.hypot(dispX2 - dispX1, dispY2 - dispY1);
+        //
+        // Offset breakdown for a 1 px visual gap between body and
+        // selection edges:
+        //   lineStrokeWidth / 2 — half of the body (centre → body edge)
+        //   0.5                 — half of the 1 px selection stroke
+        //                         (centre → selection's body-side edge)
+        //   1                   — actual gap requested between the two
+        //                         adjacent edges
+        const dx = dispX2 - dispX1;
+        const dy = dispY2 - dispY1;
+        const len = Math.hypot(dx, dy);
         if (len === 0) return null;
-        const off = lineStrokeWidth / 2 + 1;
-        const px = (-(dispY2 - dispY1) / len) * off;
-        const py = ((dispX2 - dispX1) / len) * off;
+        const off = lineStrokeWidth / 2 + 1.5;
+        const px = (-dy / len) * off;
+        const py = (dx / len) * off;
         return (
           <>
             <KLine