Implement local-first canvas sync and fix drag edge stability

lib/canvas-local-persistence.ts

@@ -151,6 +151,7 @@ export function enqueueCanvasOp(
     enqueuedAt: op.enqueuedAt ?? Date.now(),
   };
   const payload = readOpsPayload(canvasId);
+  payload.ops = payload.ops.filter((candidate) => candidate.id !== entry.id);
   payload.ops.push(entry);
   payload.updatedAt = Date.now();
   writePayload(opsKey(canvasId), payload);
@@ -166,6 +167,17 @@ export function resolveCanvasOp(canvasId: string, opId: string): void {
   writePayload(opsKey(canvasId), payload);
 }
 
+export function resolveCanvasOps(canvasId: string, opIds: string[]): void {
+  if (opIds.length === 0) return;
+  const idSet = new Set(opIds);
+  const payload = readOpsPayload(canvasId);
+  const nextOps = payload.ops.filter((op) => !idSet.has(op.id));
+  if (nextOps.length === payload.ops.length) return;
+  payload.ops = nextOps;
+  payload.updatedAt = Date.now();
+  writePayload(opsKey(canvasId), payload);
+}
+
 export function readCanvasOps(canvasId: string): CanvasPendingOp[] {
   return readOpsPayload(canvasId).ops;
 }