refactor(branch-dropdown): drop worktree parent-branch merge shortcut

Remove the parent-branch button shown beside the branch selector along
with its supporting API, command, service, model, and i18n strings.
Add migration m20260423 to drop the now-unused folder.parent_branch
column.

src/lib/tauri.ts

@@ -524,16 +524,6 @@ export async function getFolderConversation(
   return invoke("get_folder_conversation", { conversationId })
 }
 
-export async function setFolderParentBranch(
-  path: string,
-  parentBranch: string | null
-): Promise<void> {
-  return invoke("set_folder_parent_branch", {
-    path,
-    parentBranch,
-  })
-}
-
 export async function removeFolderFromHistory(path: string): Promise<void> {
   return invoke("remove_folder_from_history", { path })
 }