fix: Phase 7c - fix Touch/drag-drop types (32 -> 28 errors)

Fixed TypeScript errors where custom drag data was being stored on the
native Touch type, which doesn't allow extension.

The issue: `currentTouch` was typed as `Touch | null` but the code was
trying to store custom properties (item, source, type) on it for pending
drag operations initiated by mouse pointer events.

The fix: Created a new `PendingDragData` interface to properly type the
custom drag data being stored for mouse events.

Changes:
- src/lib/composables/drag-drop.svelte.ts:
  - Added PendingDragData interface with item, source, type
  - Changed TouchState.currentTouch from `Touch | null` to `PendingDragData | null`
  - This allows storing pending drag data without conflicting with native Touch type

Result: 32 → 28 errors (-4)

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

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

src/lib/composables/drag-drop.svelte.ts

@@ -42,13 +42,19 @@ export interface DragOperation {
 	retryCount: number
 }
 
+export interface PendingDragData {
+	item: GridItem
+	source: Position
+	type: GridItemType
+}
+
 export interface TouchState {
 	touchStartPos: { x: number; y: number } | null
 	touchStartTime: number
 	longPressTimer: number | null
 	touchThreshold: number
 	longPressDuration: number
-	currentTouch: Touch | null
+	currentTouch: PendingDragData | null
 }
 
 export interface DragDropState {