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-tauri/src/commands/folders.rs

@@ -19,6 +19,7 @@ use tauri::Manager;
 use crate::app_error::AppCommandError;
 #[cfg(feature = "tauri-runtime")]
 use crate::db::error::DbError;
+#[cfg(feature = "tauri-runtime")]
 use crate::db::service::folder_service;
 use crate::db::AppDatabase;
 use crate::models::GitCredentials;
@@ -479,40 +480,6 @@ pub async fn add_folder_to_history(
     folder_service::add_folder(&db.conn, &path).await
 }
 
-pub(crate) async fn set_folder_parent_branch_core(
-    conn: &sea_orm::DatabaseConnection,
-    path: &str,
-    parent_branch: Option<String>,
-) -> Result<(), AppCommandError> {
-    use crate::db::entities::folder;
-    use sea_orm::{ColumnTrait, EntityTrait, QueryFilter};
-    let row = folder::Entity::find()
-        .filter(folder::Column::Path.eq(path))
-        .filter(folder::Column::DeletedAt.is_null())
-        .one(conn)
-        .await
-        .map_err(|e| {
-            AppCommandError::database_error("Failed to query folder").with_detail(e.to_string())
-        })?;
-
-    if let Some(folder_model) = row {
-        folder_service::set_folder_parent_branch(conn, folder_model.id, parent_branch)
-            .await
-            .map_err(AppCommandError::from)?;
-    }
-    Ok(())
-}
-
-#[cfg(feature = "tauri-runtime")]
-#[cfg_attr(feature = "tauri-runtime", tauri::command)]
-pub async fn set_folder_parent_branch(
-    db: tauri::State<'_, AppDatabase>,
-    path: String,
-    parent_branch: Option<String>,
-) -> Result<(), AppCommandError> {
-    set_folder_parent_branch_core(&db.conn, &path, parent_branch).await
-}
-
 #[cfg(feature = "tauri-runtime")]
 #[cfg_attr(feature = "tauri-runtime", tauri::command)]
 pub async fn remove_folder_from_history(