docs: Add design document for spatial workspace architecture refactor and fix card center calculation in layout system.

frontend/src/desktop/LayoutSystem.ts

@@ -271,12 +271,17 @@ export class LayoutStore {
     getCardsInCircle(x: number, y: number, radius: number): LayoutCard[] {
         const result: LayoutCard[] = [];
         for (const card of this.items.values()) {
-            const dx = card.x - x;
-            const dy = card.y - y;
+            // Calculate center of candidate card
+            const cx = card.x + card.width / 2;
+            const cy = card.y + card.height / 2;
+
+            const dx = cx - x;
+            const dy = cy - y;
 
-            // Use squared distance to avoid sqrt
             const distSq = dx * dx + dy * dy;
-            if (distSq < radius * radius) {
+            const limit = radius;
+
+            if (distSq < limit * limit) {
                 result.push(card);
             }
         }
@@ -284,7 +289,9 @@ export class LayoutStore {
     }
 
     getStackBelow(topCard: LayoutCard): string[] {
-        const candidates = this.getCardsInCircle(topCard.x, topCard.y, topCard.innerRadius);
+        const centerX = topCard.x + topCard.width / 2;
+        const centerY = topCard.y + topCard.height / 2;
+        const candidates = this.getCardsInCircle(centerX, centerY, topCard.innerRadius);
 
         return candidates
             .filter(other => {