feat: Add spatial workspace architecture design document and refactor card drag logic to use new LayoutCard drag state management.

frontend/src/desktop/CardDragLogic.ts

@@ -1,24 +1,39 @@
 import { LayoutStore } from './LayoutSystem';
 
-const capturedCards = new Set<string>();
+export const handleDragStart = (store: LayoutStore, selection: string[], leadingId: string, offset: { x: number, y: number }) => {
+    const leadingCard = store.items.get(leadingId);
+    if (!leadingCard) return;
+
+    selection.forEach(id => {
+        const card = store.items.get(id);
+        if (!card) return;
+
+        let myOffset = offset;
+        if (id !== leadingId) {
+            myOffset = {
+                x: offset.x + (leadingCard.x - card.x),
+                y: offset.y + (leadingCard.y - card.y)
+            };
+        }
+
+        card.beginDrag(myOffset);
+    });
+};
 
 export const handleDragMove = (store: LayoutStore, selection: string[], delta: { x: number, y: number }) => {
     selection.forEach(id => {
         const card = store.items.get(id);
         if (card) {
-            if (!capturedCards.has(id)) {
-                card.captureDragStart();
-                capturedCards.add(id);
-            }
-
-            card.update({
-                x: card.dragStartX + delta.x,
-                y: card.dragStartY + delta.y
-            });
+            card.continueDrag(delta);
         }
     });
 };
 
-export const handleDragEnd = () => {
-    capturedCards.clear();
+export const handleDragEnd = (store: LayoutStore, selection: string[]) => {
+    selection.forEach(id => {
+        const card = store.items.get(id);
+        if (card) {
+            card.finishDrag();
+        }
+    });
 };

frontend/src/desktop/LayoutSystem.ts

@@ -25,6 +25,7 @@ export class LayoutCard implements LayoutCardState {
     public store: LayoutStore;
     public dragStartX: number = 0;
     public dragStartY: number = 0;
+    private dragOffset: { x: number, y: number } | null = null;
 
     constructor(id: string, store: LayoutStore, initialData: Partial<LayoutCardState> = {}, ref: HTMLElement | null = null) {
         this.id = id;
@@ -39,11 +40,28 @@ export class LayoutCard implements LayoutCardState {
         this.applyTransform();
     }
 
-    captureDragStart() {
+    beginDrag(offset: { x: number, y: number }) {
+        this.dragOffset = offset;
         this.dragStartX = this.x;
         this.dragStartY = this.y;
     }
 
+    continueDrag(delta: { x: number, y: number }) {
+        if (!this.dragOffset) return;
+
+        const newX = this.x + delta.x;
+        const newY = this.y + delta.y;
+
+        this.update({
+            x: newX,
+            y: newY
+        });
+    }
+
+    finishDrag() {
+        this.dragOffset = null;
+    }
+
     update(changes: Partial<LayoutCardState>) {
         Object.assign(this, changes);
         if (changes.width !== undefined || changes.height !== undefined) {

frontend/src/desktop/useCardPointer.ts

@@ -1,7 +1,7 @@
 import React, { useCallback, useRef } from 'react';
 
 import { LayoutCard } from './LayoutSystem';
-import { handleDragMove, handleDragEnd } from './CardDragLogic';
+import { handleDragMove, handleDragEnd, handleDragStart } from './CardDragLogic';
 
 const DRAG_THRESHOLD = 3;
 
@@ -17,6 +17,7 @@ export const useCardPointer = (
 ) => {
     const [state, setState] = React.useState<PointerState>('idle');
     const initialPosition = useRef<{ x: number, y: number } | null>(null);
+    const lastPosition = useRef<{ x: number, y: number } | null>(null);
 
     const updateState = useCallback((e: React.PointerEvent) => {
         if (state === 'click' && initialPosition.current) {
@@ -37,6 +38,7 @@ export const useCardPointer = (
         (e.target as Element).setPointerCapture(e.pointerId);
         setState('click');
         initialPosition.current = { x: e.clientX, y: e.clientY };
+        lastPosition.current = { x: e.clientX, y: e.clientY };
     }, []);
 
     const onPointerMove = useCallback((e: React.PointerEvent) => {
@@ -44,24 +46,45 @@ export const useCardPointer = (
         updateState(e);
 
         if (state === 'drag-start') {
+            let effectiveSelection = selection;
             if (!isSelected) {
                 onSelect([card.id], hasModifier);
-
-                if (!hasModifier) {
-                    card.bringToFront();
+                if (hasModifier) {
+                    effectiveSelection = [...selection, card.id];
+                } else {
+                    effectiveSelection = [card.id];
                 }
             }
 
+            card.bringToFront();
+
             setState('drag');
+
+            if (initialPosition.current) {
+                const rect = card.ref.getBoundingClientRect();
+                const centerX = rect.left + rect.width / 2;
+                const centerY = rect.top + rect.height / 2;
+                const offset = {
+                    x: initialPosition.current.x - centerX,
+                    y: initialPosition.current.y - centerY
+                };
+
+                handleDragStart(card.store, effectiveSelection, card.id, offset);
+                // Reset lastPosition to current pointer to avoid jump on first move
+                lastPosition.current = { x: e.clientX, y: e.clientY };
+            }
         }
 
-        if (state === 'drag' && initialPosition.current) {
+        if (state === 'drag' && lastPosition.current) {
             const delta = {
-                x: e.clientX - initialPosition.current.x,
-                y: e.clientY - initialPosition.current.y
+                x: e.clientX - lastPosition.current.x,
+                y: e.clientY - lastPosition.current.y
             };
 
-            handleDragMove(card.store, selection, delta);
+            if (delta.x !== 0 || delta.y !== 0) {
+                handleDragMove(card.store, selection, delta);
+                lastPosition.current = { x: e.clientX, y: e.clientY };
+            }
         }
     }, [card, state, updateState, isSelected, selection, onSelect]);
 
@@ -70,7 +93,7 @@ export const useCardPointer = (
         updateState(e);
 
         if (state === 'drag' || state === 'drag-start') {
-            handleDragEnd();
+            handleDragEnd(card.store, selection);
         } else if (state === 'click') {
             if (isSelected) {
                 const isUnobstructed = card.isUnobstructed();
@@ -91,8 +114,9 @@ export const useCardPointer = (
 
         setState('idle');
         initialPosition.current = null;
+        lastPosition.current = null;
         (e.target as Element).releasePointerCapture(e.pointerId);
-    }, [card, state, isSelected, onSelect, onDeselect, onDocumentActivate, updateState]);
+    }, [card, state, isSelected, onSelect, onDeselect, onDocumentActivate, updateState, selection]);
 
     return {
         onPointerDown,