refactor(web): extract allocation multi-drag helpers

apps/web/src/hooks/useTimelineDrag.ts

@@ -17,6 +17,12 @@ import {
   requiresAllocationFragmentExtraction,
   shouldTreatAllocationDragAsClick,
 } from "./timelineAllocationFinalize.js";
+import {
+  finalizeAllocationMultiDrag,
+  isAllocationMultiSelected,
+  startAllocationMultiDrag,
+  updateAllocationMultiDrag,
+} from "./timelineAllocationMultiDrag.js";
 import {
   createMultiSelectState,
   finalizeMultiSelectDraft,
@@ -648,28 +654,26 @@ export function useTimelineDrag({
 
       // Check if this allocation is part of a multi-selection → multi-drag mode
       const ms = multiSelectRef.current;
-      const isMultiSelected =
-        ms.selectedAllocationIds.length > 1 &&
-        ms.selectedAllocationIds.includes(opts.allocationId);
+      const isMultiSelected = isAllocationMultiSelected(ms, opts.allocationId);
 
       if (isMultiSelected) {
         // ── Multi-drag: move/resize all selected allocations together ──
         const startMouseX = e.clientX;
-        let currentDaysDelta = 0;
         const dragMode = opts.mode;
+        const initialMultiDragState = startAllocationMultiDrag(ms, dragMode);
 
-        setMultiSelectState((prev) => ({ ...prev, isMultiDragging: true, multiDragDaysDelta: 0, multiDragMode: dragMode }));
-        multiSelectRef.current = { ...ms, isMultiDragging: true, multiDragDaysDelta: 0, multiDragMode: dragMode };
+        setMultiSelectState(initialMultiDragState);
+        multiSelectRef.current = initialMultiDragState;
         multiSelectCleanupRef.current?.();
 
         function handleMultiMove(ev: MouseEvent) {
           const deltaX = ev.clientX - startMouseX;
           const daysDelta = pixelsToDays(deltaX, cellWidthRef.current);
-          if (daysDelta === currentDaysDelta) return;
-          currentDaysDelta = daysDelta;
+          const updated = updateAllocationMultiDrag(multiSelectRef.current, daysDelta);
+          if (!updated) return;
 
-          setMultiSelectState((prev) => ({ ...prev, multiDragDaysDelta: daysDelta }));
-          multiSelectRef.current = { ...multiSelectRef.current, multiDragDaysDelta: daysDelta };
+          setMultiSelectState(updated);
+          multiSelectRef.current = updated;
         }
 
         function handleMultiUp(ev: MouseEvent) {
@@ -677,13 +681,14 @@ export function useTimelineDrag({
           multiSelectCleanupRef.current = null;
 
           const finalDelta = pixelsToDays(ev.clientX - startMouseX, cellWidthRef.current);
+          const finalized = finalizeAllocationMultiDrag(multiSelectRef.current);
 
-          setMultiSelectState((prev) => ({ ...prev, isMultiDragging: false, multiDragDaysDelta: 0 }));
-          multiSelectRef.current = { ...multiSelectRef.current, isMultiDragging: false, multiDragDaysDelta: 0 };
+          setMultiSelectState(finalized);
+          multiSelectRef.current = finalized;
 
           if (finalDelta !== 0) {
             // Pass IDs from ref to avoid stale closure in the callback
-            const ids = multiSelectRef.current.selectedAllocationIds;
+            const ids = finalized.selectedAllocationIds;
             onMultiDragCompleteRef.current?.(finalDelta, dragMode, ids);
           }
         }