refactor(web): extract allocation multi-drag session

apps/web/src/hooks/useTimelineDrag.ts

@@ -18,6 +18,7 @@ import {
   startAllocationMultiDrag,
   updateAllocationMultiDrag,
 } from "./timelineAllocationMultiDrag.js";
+import { beginAllocationMultiDragSession } from "./timelineAllocationMultiDragSession.js";
 import { resolveAllocationRelease } from "./timelineAllocationRelease.js";
 import { createAllocationDragState } from "./timelineAllocationDragState.js";
 import { cleanupTimelineDragState } from "./timelineDragCleanup.js";
@@ -593,43 +594,22 @@ export function useTimelineDrag({
       const isMultiSelected = isAllocationMultiSelected(ms, opts.allocationId);
 
       if (isMultiSelected) {
-        // ── Multi-drag: move/resize all selected allocations together ──
-        const startMouseX = e.clientX;
-        const dragMode = opts.mode;
-        const initialMultiDragState = startAllocationMultiDrag(ms, dragMode);
-
-        setMultiSelectState(initialMultiDragState);
-        multiSelectRef.current = initialMultiDragState;
-        multiSelectCleanupRef.current?.();
-
-        function handleMultiMove(ev: MouseEvent) {
-          const deltaX = ev.clientX - startMouseX;
-          const daysDelta = pixelsToDays(deltaX, cellWidthRef.current);
-          const updated = updateAllocationMultiDrag(multiSelectRef.current, daysDelta);
-          if (!updated) return;
-
-          setMultiSelectState(updated);
-          multiSelectRef.current = updated;
-        }
-
-        function handleMultiUp(ev: MouseEvent) {
-          multiSelectCleanupRef.current?.();
-          multiSelectCleanupRef.current = null;
-
-          const finalDelta = pixelsToDays(ev.clientX - startMouseX, cellWidthRef.current);
-          const finalized = finalizeAllocationMultiDrag(multiSelectRef.current);
-
-          setMultiSelectState(finalized);
-          multiSelectRef.current = finalized;
-
-          if (finalDelta !== 0) {
-            // Pass IDs from ref to avoid stale closure in the callback
-            const ids = finalized.selectedAllocationIds;
-            onMultiDragCompleteRef.current?.(finalDelta, dragMode, ids);
-          }
-        }
-
-        multiSelectCleanupRef.current = attachDocumentMouseDrag(document, handleMultiMove, handleMultiUp);
+        beginAllocationMultiDragSession({
+          startMouseX: e.clientX,
+          dragMode: opts.mode,
+          documentTarget: document,
+          cleanupRef: multiSelectCleanupRef,
+          stateRef: multiSelectRef,
+          setState: setMultiSelectState,
+          startState: startAllocationMultiDrag,
+          updateState: updateAllocationMultiDrag,
+          finalizeState: finalizeAllocationMultiDrag,
+          toDaysDelta: (deltaX) => pixelsToDays(deltaX, cellWidthRef.current),
+          onComplete: (daysDelta, finalized) => {
+            onMultiDragCompleteRef.current?.(daysDelta, opts.mode, finalized.selectedAllocationIds);
+          },
+          attachDrag: attachDocumentMouseDrag,
+        });
         return;
       }