Set default branch to main for clone creation

When creating a clone without specifying a branch, both the UI and API
now default to "main" branch instead of using the first available
branch or the snapshot's branch.

UI: Changed to prioritize "main" branch when selecting default.
API: Changed default from snapshot.Branch to branching.DefaultBranch.

Related: https://gitlab.com/postgres-ai/database-lab/-/issues/662

Author: claude

engine/internal/cloning/base.go

@@ -30,6 +30,7 @@ import (
 	"gitlab.com/postgres-ai/database-lab/v3/pkg/log"
 	"gitlab.com/postgres-ai/database-lab/v3/pkg/models"
 	"gitlab.com/postgres-ai/database-lab/v3/pkg/util"
+	"gitlab.com/postgres-ai/database-lab/v3/pkg/util/branching"
 	"gitlab.com/postgres-ai/database-lab/v3/pkg/util/pglog"
 )
 
@@ -172,7 +173,7 @@ func (c *Base) CreateClone(cloneRequest *types.CloneCreateRequest) (*models.Clon
 	}
 
 	if cloneRequest.Branch == "" {
-		cloneRequest.Branch = snapshot.Branch
+		cloneRequest.Branch = branching.DefaultBranch
 	}
 
 	clone := &models.Clone{

ui/packages/shared/pages/CreateClone/index.tsx

@@ -103,7 +103,8 @@ export const CreateClone = observer((props: Props) => {
 
       const branches = (await stores.main.getBranches(props.instanceId)) ?? []
 
-      let initiallySelectedBranch = branches[0]?.name;
+      const mainBranch = branches.find((branch) => branch.name === 'main')
+      let initiallySelectedBranch = mainBranch?.name ?? branches[0]?.name;
 
       if (initialBranch && branches.find((branch) => branch.name === initialBranch)) {
         initiallySelectedBranch = initialBranch;