feat(worktree): add start point option for new branch creation

Allow selecting a base branch when creating new worktree branches
Add dropdown to choose start point from existing branches
Extend worktree creation flow to support custom branch origins

packages/ui/src/lib/api/types.ts

@@ -222,6 +222,7 @@ export interface GitAddWorktreePayload {
   path: string;
   branch: string;
   createBranch?: boolean;
+  startPoint?: string;
 }
 
 export interface GitRemoveWorktreePayload {