fix(flashcards): keep grab point under cursor with proper coordinate conversion

Fixed the issue where cards would slip out from under the cursor when rotating.
The grab point now stays perfectly under your cursor throughout the drag.

The problem: Simple delta positioning didn't account for rotation causing the
grab point to rotate away from the cursor position.

The solution: Properly convert between coordinate systems:
1. Calculate rotated grab offset (2D rotation matrix)
2. Determine card center: cursor position - rotated grab offset (viewport space)
3. Convert from viewport coordinates to container coordinates
4. Calculate top-left position from center for CSS translate()

Key changes:
- Pass containerRef down to DraggableCard components
- Get container bounds for viewport→container conversion
- Apply rotation matrix to grab offset
- Position card so rotated grab point aligns with cursor

Now when you grab a card by its edge and drag, the exact point you grabbed
stays glued to your cursor while the card rotates around its center.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/components/InteractiveFlashcards.tsx

@@ -81,7 +81,7 @@ export function InteractiveFlashcards() {
       })}
     >
       {cards.map((card) => (
-        <DraggableCard key={card.id} card={card} />
+        <DraggableCard key={card.id} card={card} containerRef={containerRef} />
       ))}
     </div>
   )
@@ -89,9 +89,10 @@ export function InteractiveFlashcards() {
 
 interface DraggableCardProps {
   card: Flashcard
+  containerRef: React.RefObject<HTMLDivElement>
 }
 
-function DraggableCard({ card }: DraggableCardProps) {
+function DraggableCard({ card, containerRef }: DraggableCardProps) {
   // Track position - starts at initial, updates when dragged
   const [position, setPosition] = useState({ x: card.initialX, y: card.initialY })
   const [rotation, setRotation] = useState(card.initialRotation) // Now dynamic!
@@ -206,11 +207,46 @@ function DraggableCard({ card }: DraggableCardProps) {
       )
     }
 
-    // Update card position - simple delta from drag start
-    // The rotation is visual only and happens around the card's center via CSS transform-origin
+    // Update card position - keep grab point under cursor while rotating
+    // Calculate the rotated grab offset
+    const rotationRad = (clampedRotation * Math.PI) / 180
+    const cosRot = Math.cos(rotationRad)
+    const sinRot = Math.sin(rotationRad)
+
+    // Rotate the grab offset by the current rotation angle
+    const rotatedGrabX = grabOffsetRef.current.x * cosRot - grabOffsetRef.current.y * sinRot
+    const rotatedGrabY = grabOffsetRef.current.x * sinRot + grabOffsetRef.current.y * cosRot
+
+    // Get container bounds for coordinate conversion
+    if (!containerRef.current || !cardRef.current) {
+      // Fallback to simple delta if refs not ready
+      setPosition({
+        x: dragStartRef.current.cardX + deltaX,
+        y: dragStartRef.current.cardY + deltaY,
+      })
+      return
+    }
+
+    const containerRect = containerRef.current.getBoundingClientRect()
+    const cardRect = cardRef.current.getBoundingClientRect()
+
+    // Current cursor position in viewport space
+    const cursorViewportX = e.clientX
+    const cursorViewportY = e.clientY
+
+    // Card center should be at: cursor - rotated grab offset (viewport space)
+    const cardCenterViewportX = cursorViewportX - rotatedGrabX
+    const cardCenterViewportY = cursorViewportY - rotatedGrabY
+
+    // Convert card center from viewport space to container space
+    const cardCenterContainerX = cardCenterViewportX - containerRect.left
+    const cardCenterContainerY = cardCenterViewportY - containerRect.top
+
+    // position.x/y represents translate() which positions the top-left corner
+    // So we need: top-left = center - (width/2, height/2)
     setPosition({
-      x: dragStartRef.current.cardX + deltaX,
-      y: dragStartRef.current.cardY + deltaY,
+      x: cardCenterContainerX - cardRect.width / 2,
+      y: cardCenterContainerY - cardRect.height / 2,
     })
   }